Iron-based oxide magnetic powder and method for producing same

ABSTRACT

A raw material solution containing trivalent iron ions, or trivalent iron ions and ions of a metal element that partially substitutes Fe sites, and an alkaline aqueous solution for neutralizing the raw material solution are added to a reaction system to adjust the pH of the reaction system from 1.0 to 3.0 or lower. Hydroxycarboxylic acid is added to the obtained reaction solution and the pH of the reaction system is then neutralized from 7.0 to 10.0 or lower. The obtained precipitate of a substituent metal element-containing iron oxyhydroxide is coated with silicon oxide, followed by heating so as to form particles of ε-iron oxide in which Fe sites are partially substituted by other metal elements, and then, a slurry containing the particles is classified. The iron-based oxide magnetic powder has a particle shape close to a perfect sphere and is suitable for use in a magnetic recording medium.

TECHNICAL FIELD

The present invention relates to an iron-based oxide magnetic powdersuitable for a high-density magnetic recording medium, a radio waveabsorber, etc., and particularly to a magnetic powder in which theaverage particle diameter of the particles is on the order ofnanometers, and a method for producing the same.

BACKGROUND ART

While ε-Fe₂O₃ is an extremely rare phase among iron oxides, particlesthereof having a size on the order of nanometers show a great coerciveforce (Hc) of about 20 kOe (1.59×10⁶ A/m) at room temperature, andtherefore, a production method for synthesizing ε-Fe₂O₃ as a singlephase has been conventionally studied (PTL 1). Further, when ε-Fe₂O₃ isused in a magnetic recording medium, there is currently no material fora magnetic head having a high level of saturation magnetic flux densitycorresponding thereto, and therefore, the adjustment of the coerciveforce is also performed by partially substituting the Fe sites ofε-Fe₂O₃ by a trivalent metal such as Al, Ga, or In, and the relationshipbetween the coercive force and the radio wave absorption properties hasalso been examined (PTL 2).

On the other hand, in the field of magnetic recording, development of amagnetic recording medium having a high ratio of reproduced signal levelto particulate noise (C/N ratio: carrier to noise ratio) has beenconducted, and in order to increase the recording density, there hasbeen a demand for refinement of magnetic particles constituting amagnetic recording layer. However, the refinement of magnetic particlesis generally likely to cause deterioration of the environmentalstability and thermal stability thereof, and there is a concern aboutthe deterioration of the magnetic properties of the magnetic particlesunder the use or storage environment. Therefore, by partiallysubstituting the Fe sites of ε-Fe₂O₃ by another metal having excellentheat resistance, various types of partially substituted ε-Fe₂O₃, inwhich the particle size is reduced and the coercive force is madevariable, and which also have excellent environmental stability andthermal stability, represented by a general formulaε-A_(x)B_(y)Fe_(2-x-y)O₃ or ε-A_(x)B_(y)C_(z)Fe_(2-x-y-z)O₃ (wherein Ais a divalent metal element such as Co, Ni, Mn, or Zn, B is atetravalent metal element such as Ti, and C is a trivalent metal elementsuch as In, Ga, or Al) have been developed (PTL 3).

ε-Fe₂O₃ is obtained as a stable phase with a size on the order ofnanometers, and therefore, a special method is needed for itsproduction. In the above-mentioned PTLs 1 to 3, a method for producingε-Fe₂O₃, in which a fine crystal of an iron oxyhydroxide produced by aliquid-phase method is used as a precursor, and the precursor is coatedwith silicon oxide by a sol-gel method, followed by a heat treatment, isdisclosed, and as the liquid-phase method, each of a reverse micellemethod using an organic solvent as a reaction medium and a method usingonly an aqueous solution as a reaction medium is disclosed. However, inε-Fe₂O₃ or partially substituted ε-Fe₂O₃ obtained by such a method,there are variations in magnetic properties, and therefore, improvementof the magnetic properties thereof by removing the silicon oxide coatingafter the heat treatment, and performing a classification treatment hasbeen proposed.

ε-Fe₂O₃ or an ε-type iron-based oxide in which Fe is partiallysubstituted produced by a conventional production method disclosed inthe above-mentioned PTLs 1 to 3 has excellent magnetic properties,however, depending on the production conditions, variations aresometimes observed in a coercive force distribution. In response to theproblem of variations in coercive force distribution, PTL 4 discloses amethod for producing an ε-type iron-based oxide magnetic particle powdercontaining a substituent metal element, in which an alkali is added toan aqueous solution containing trivalent iron ions and ions of a metal Mthat partially substitutes the Fe sites so as to neutralize the pH to1.0 or higher and 3.0 or lower, and thereafter a hydroxycarboxylic acidD is added thereto in an amount of 0.125 or more and 1.0 or less interms of molar ratio with respect to the amount of the trivalent ironions: D/Fe or molar ratio with respect to the total amount of thetrivalent iron ions and the ions of the substituent metal M: (D/(Fe+M)),and an alkali is further added thereto so as to neutralize the pH to 7.0or higher and 10.0 or lower, and the produced iron oxyhydroxidecontaining the substituent metal element is coated with silicon oxide,followed by heating as a method for producing an ε-type iron-based oxidemagnetic particle powder containing a substituent metal element. Thehydroxycarboxylic acid which is a carboxylic acid having an OH group inthe molecule is assumed to bring about an effect of forming a complexwith the trivalent iron ion dissolved in the reaction solution, anddelaying a hydroxide formation reaction of iron when an alkali isfurther added in the subsequent step resulting in narrowing thedistribution of the average particle diameter of precursor fineparticles containing an iron oxyhydroxide to be produced.

CITATION LIST Patent Literature

-   PTL 1: JP-A-2008-174405-   PTL 2: WO 2008/029861-   PTL 3: WO 2008/149785-   PTL 4: JP-A-2017-024981

SUMMARY OF INVENTION Technical Problem

The ε-type iron-based oxide magnetic powder containing a substituentmetal element produced by the production method disclosed in PTL 4 wasexcellent in that the particle size distribution is narrow and thecontent of fine particles that do not contribute to magnetic recordingproperties is low so that the coercive force distribution is narrow, butthere was still room for improvement for increasing the recordingdensity of a magnetic recording medium.

When applying the ε-type iron-based oxide magnetic powder to a magneticrecording medium, it is desired that not only the average particlediameter is merely controlled, but also the abundance ratio of fineparticles or coarse particles whose particle diameter is far from theaverage value is decreased as much as possible. The presence of fineparticles causes an increase in noise in the electromagnetic conversionproperty SNR of the magnetic recording medium even if the volume ratioto the magnetic powder is small. Further, the coarse particles areparticles whose coercive force is too high to perform writing with amagnetic head, and if the abundance ratio thereof is large, it causes adecrease in the magnetic recording density. The ε-type iron-based oxidemagnetic powder containing a substituent metal element produced by theproduction method disclosed in PTL 4 cannot be said to have asufficiently reduced content of fine particles and coarse particlesdescribed above, and still had a problem with an increase in therecording density of a magnetic recording medium.

Further, the ε-type iron-based oxide magnetic powder containing asubstituent metal element produced by the production method disclosed inPTL 4 also had a problem that the particle shape is distorted. If theparticle shape is distorted, when the particles are magneticallyoriented during the production of a magnetic recording medium, theprotruding and recessed parts of the particles may interfere with eachother and hinder the orientation of the particles, and therefore, it isdesired that the particle shape is as close to a perfect sphere aspossible.

In view of the above problems, in the present invention, an iron-basedoxide magnetic powder that is composed of particles of ε-iron oxide inwhich Fe sites are partially substituted by other metal elements, thathas a sufficiently reduced content of fine particles and coarseparticles, that has a particle shape close to a perfect sphere, and thatis suitable for use in a magnetic recording medium, and a method forproducing an iron-based oxide magnetic powder are provided.

Solution to Problem

In order to solve the above problems, in the present invention,

(1) an iron-based oxide magnetic powder composed of particles of ε-ironoxide in which Fe sites are partially substituted by other metalelements, wherein an average particle diameter measured with atransmission electron microscope is 10 nm or more and 20 nm or less, thenumber ratio of particles with a particle diameter of 8 nm or less is 3%or less, the number ratio of particles with a particle diameter of 20 nmor more is 25% or less, the average circularity of particles measuredwith a transmission electron microscope is 0.955 or more, and thecoefficient of variation of the particle diameter measured with atransmission electron microscope is 19% or less is provided.

(2) Further, it is preferred that the metal element that partiallysubstitutes the Fe sites is one type or two or more types of Ga, Co, andTi.

(3) It does not matter that the metal element that partially substitutesthe Fe sites is one type or two or more types of Ga, Co, Ti, Ni, Mn, Cr,Nd, Dy, and Gd.

In the present invention, further,

(4) a method for producing an iron-based oxide magnetic powder, whichhas an average particle diameter measured with a transmission electronmicroscope of 10 nm or more and 20 nm or less, and is composed ofparticles of ε-iron oxide or ε-iron oxide in which Fe sites arepartially substituted by other metal elements, including:

a raw material solution preparation step of preparing an aqueoussolution containing trivalent iron ions, or trivalent iron ions and ionsof a metal element that partially substitutes the Fe sites (hereinafterreferred to as a raw material solution), and an alkaline aqueoussolution for neutralizing the raw material solution;

a first neutralization step of continuously or intermittently addingeach of the raw material solution and the alkaline aqueous solution to areaction system and mixing so as to adjust the pH of the reaction systemto 1.0 or higher and 3.0 or lower;

a step of adding a hydroxycarboxylic acid to the aqueous solution afterthe first neutralization step;

a second neutralization step of neutralizing the pH to 7.0 or higher and10.0 or lower by adding an alkali to the aqueous solution to which thehydroxycarboxylic acid is added, thereby obtaining a slurry containing aprecipitate of an iron oxyhydroxide or a substituent metalelement-containing iron oxyhydroxide;

a step of adding a silicon compound having a hydrolyzable group to theslurry containing the iron oxyhydroxide or the substituent metalelement-containing iron oxyhydroxide, thereby coating the ironoxyhydroxide or the substituent metal element-containing ironoxyhydroxide with a hydrolysate of the silicon compound;

a step of heating the iron oxyhydroxide or the substituent metalelement-containing iron oxyhydroxide coated with the hydrolysate of thesilicon compound, thereby forming ε-iron oxide or ε-iron oxide, in whichFe sites are partially substituted by other metal elements, coated withsilicon oxide;

a step of removing the silicon oxide on the ε-iron oxide or the ε-ironoxide, in which Fe sites are partially substituted by other metalelements, coated with the silicon oxide, thereby obtaining a slurrycontaining the ε-iron oxide or the ε-iron oxide, in which Fe sites arepartially substituted by other metal elements;

a step of adding a quaternary ammonium salt at a concentration of 0.009mol/kg or more and 1.0 mol/kg or less as a surface modifier to theslurry containing the ε-iron oxide or the ε-iron oxide, in which Fesites are partially substituted by other metal elements, and alsoadjusting the pH to 11.0 or higher and 14.0 or lower;

a step of subjecting the surface modifier-containing slurry to adispersion treatment, thereby obtaining a dispersion slurry of particlesof the ε-iron oxide or the ε-iron oxide, in which Fe sites are partiallysubstituted by other metal elements; and

a step of classifying the dispersion slurry of the particles of theε-iron oxide or the ε-iron oxide, in which Fe sites are partiallysubstituted by other metal elements is provided.

(5) It is preferred that the first neutralization step described in theabove item (4) is a step of continuously or intermittently adding eachof the raw material solution and the alkaline aqueous solutioncontaining the alkali in an amount of 0.4 equivalents or more and 0.9equivalents or less with respect to the total amount of an acid groupcontained in the raw material solution to the reaction system which doesnot contain trivalent iron ions or ions of a metal element thatpartially substitutes the Fe sites and mixing so as to adjust the pH ofthe reaction system to 1.0 or higher and 3.0 or lower.

(6) The first neutralization step described in the above item (4) may bea step in which when each of the raw material solution and the alkalineaqueous solution is continuously or intermittently added to the reactionsystem which previously contains trivalent iron ions, or trivalent ironions and ions of a metal element that partially substitutes the Fesites, the amount of the trivalent iron ions and the ions of the metalelement that partially substitutes the Fe sites previously contained inthe reaction system is set to 50 mol % or less of the sum of the amountof the trivalent iron ions and the ions of the metal element thatpartially substitutes the Fe sites and the amount of the trivalent ironions and the ions of the metal element that partially substitutes the Fesites contained in the raw material solution added to the reactionsystem, and each of the raw material solution and the alkaline aqueoussolution containing the alkali in an amount of 0.4 equivalents or moreand 1.8 equivalents or less with respect to the total amount of an acidgroup contained in the raw material solution is continuously orintermittently added to the reaction system and mixed, so as to adjustthe pH of the reaction system to 1.0 or higher and 3.0 or lower.

(7) It is preferred that in the first neutralization step in the aboveitem (5), the addition rate of each of the raw material solution and thealkaline aqueous solution is adjusted so as to maintain the cumulativeamount of the addition amount of the alkaline aqueous solution withrespect to the total cumulative addition amount of the acid groupcontained in the raw material solution within a range of 0.4 equivalentsor more and 0.9 equivalents or less through the step.

(8) It is preferred that in the first neutralization step in the aboveitem (4), the raw material solution and the alkaline aqueous solutionare added over 10 minutes or more.

(9) It is preferred that the average diameter measured with a dynamiclight scattering particle size distribution analyzer of the slurrycontaining the iron oxyhydroxide or the substituent metalelement-containing iron oxyhydroxide obtained in the secondneutralization step in the above item (4) is 300 nm or less.

(10) It is preferred that iron(III) chloride is used as a supply sourceof the trivalent iron ions contained in the raw material solution in theabove item (4).

(11) It is preferred that the cumulative addition amount of the alkaliwith respect to the total cumulative addition amount of the acid groupcontained in the raw material solution in the above item (5) is set to0.6 equivalents or more and 0.9 equivalents or less through the firstneutralization step.

(12) It is preferred that the cumulative addition amount of the alkaliwith respect to the total cumulative addition amount of the acid groupcontained in the raw material solution in the above item (6) is set to0.4 equivalents or more and 1.8 equivalents or less through the firstneutralization step.

Advantageous Effects of Invention

By using the production method of the present invention, an iron-basedoxide magnetic powder, which has a sufficiently reduced content of fineparticles and coarse particles that do not contribute to the improvementof magnetic recording properties, has a particle shape close to aperfect sphere, and is suitable for use in a magnetic recording mediumcan be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a TEM photograph of iron oxyhydroxide crystals containingsubstituent elements obtained in Example 2.

FIG. 2 is a TEM photograph of an iron-based oxide magnetic powderobtained in Example 2.

FIG. 3 is a TEM photograph of an iron-based oxide magnetic powderobtained in Example 3.

FIG. 4 is a TEM photograph of an iron-based oxide magnetic powderobtained in Example 4.

FIG. 5 is a TEM photograph of an iron-based oxide magnetic powderobtained in Comparative Example 1.

FIG. 6 is a TEM photograph of iron oxyhydroxide crystals containingsubstituent elements obtained in Comparative Example 2.

FIG. 7 is a TEM photograph of an iron-based oxide magnetic powderobtained in Comparative Example 2.

DESCRIPTION OF EMBODIMENTS [Iron-Based Oxide Magnetic Powder]

An iron-based oxide magnetic powder obtained according to the presentinvention is composed of particles of ε-iron oxide or particles ofε-iron oxide in which Fe sites are partially substituted by other metalelements (hereinafter sometimes referred to as substitution-type ε-ironoxide particles). The magnetic powder sometimes contains a differentphase (for example, one in which the Fe sites of α-Fe₂O₃ are partiallysubstituted by other metal elements) that is unavoidable in theproduction. It is possible to confirm that the magnetic powder iscomposed of particles of ε-iron oxide in which Fe sites are partiallysubstituted by other metal elements using X-ray diffractometry (XRD),high-energy electron diffractometry (HEED), or the like.

Examples of the substitution-type ε-iron oxide particles that can beproduced by the production method of the present invention include thefollowing.

One represented by a general formula: ε-C_(z)Fe_(2-z)O₃ (wherein C isone or more types of trivalent metal elements selected from In, Ga, Al,Cr, Nd, Dy, and Gd)

One represented by a general formula: ε-A_(x)B_(y)Fe_(2-x-y)O₃ (whereinA is one or more types of divalent metal elements selected from Co, Ni,Mn, and Zn, and B is one or more types of tetravalent metal elementsselected from Ti and Sn)

One represented by a general formula: ε-A_(x)C_(z)Fe_(2-x-z)O₃ (whereinA is one or more types of divalent metal elements selected from Co, Ni,Mn, and Zn, and C is one or more types of trivalent metal elementsselected from In, Ga, Al, Cr, Nd, Dy, and Gd)

One represented by a general formula: ε-B_(y)C_(z)Fe_(2-y-z)O₃ (whereinB is one or more types of tetravalent metal elements selected from Tiand Sn, and C is one or more types of trivalent metal elements selectedfrom In, Ga, Al, Cr, Nd, Dy, and Gd)

One represented by a general formula: ε-A_(x)B_(y)C_(z)Fe_(2-x-y-z)O₃(wherein A is one or more types of divalent metal elements selected fromCo, Ni, Mn, and Zn, B is one or more types of tetravalent metal elementsselected from Ti and Sn, and C is one or more types of trivalent metalelements selected from In, Ga, Al, Cr, Nd, Dy, and Gd)

The metal element that partially substitutes the Fe sites is preferablyone type or two or more types of Ga, Co, and Ti from the viewpoint ofbalance between magnetic properties and thermal stability. In addition,as the substituent element, in addition to Ga, Co, and Ti, one type ortwo or more types of Ni, Mn, Cr, Nd, Dy, and Gd can be used.

[Particle Diameter]

It is preferred that the ε-iron oxide particles or the substitution-typeε-iron oxide particles that constitute the iron-based oxide magneticpowder of the present invention have an average particle diametermeasured with a transmission electron microscope (hereinafter sometimesreferred to as TEM average particle diameter) of 10 nm or more and 20 nmor less. When the TEM average particle diameter is 20 nm or less, theelectromagnetic conversion property when the iron-based oxide magneticpowder is used in a magnetic recording medium can be improved. The TEMaverage particle diameter is preferably 17 nm or less. When the TEMaverage particle diameter of the substitution-type ε-iron oxideparticles decreases, the abundance ratio of fine particles that do notcontribute to the improvement of magnetic properties increases todeteriorate the magnetic properties per unit mass of the magneticpowder, and therefore, the TEM average particle diameter is preferably10 nm or more. Note that in the present description, the particlediameter measured with a transmission electron microscope refers to aparticle diameter of a primary particle of ε-iron oxide in which Fesites are partially substituted.

The iron-based oxide magnetic powder of the present invention ischaracterized in that with respect to the ε-iron oxide particles or thesubstitution-type ε-iron oxide particles, the number ratio of particleswith a particle diameter measured with a transmission electronmicroscope (hereinafter sometimes referred to as TEM particle diameter)of 8 nm or less is 3% or less, and the number ratio of particles with aTEM particle diameter of 20 nm or more is 25% or less.

The n-iron oxide particles or the substitution-type ε-iron oxideparticles with a TEM particle diameter of 8 nm or less have a lowcoercive force, and therefore are particles that do not contribute tomagnetic recording even if they are present in a magnetic recordingmedium, but on the contrary are particles that may lead to an increasein noise components in the electromagnetic conversion property of themagnetic recording medium. Therefore, in the ε-iron oxide particles orthe substitution-type ε-iron oxide particles, it is preferred that thenumber ratio of particles with a TEM particle diameter of 8 nm or lessis as small as possible. In the iron-based oxide magnetic powder of thepresent invention, the number ratio of particles with a TEM particlediameter of 8 nm or less is 3% or less, and therefore, the amount ofnoise components in the electromagnetic conversion property when theiron-based oxide magnetic powder is used in a magnetic recording mediumcan be decreased as compared with the case where a conventionaliron-based oxide magnetic powder is used.

Further, in the iron-based oxide magnetic powder composed of the ε-ironoxide particles or the substitution-type ε-iron oxide particles having aTEM average particle diameter of 10 nm or more and 20 nm or less, theε-iron oxide particles or the substitution-type ε-iron oxide particleswith a TEM particle diameter of 20 nm or more have a larger coerciveforce than the particles with an average particle diameter, andtherefore, even if such particles are present in a magnetic recordingmedium, a magnetic record cannot be written by a magnetic head, andtherefore, such particles also do not contribute to magnetic recording,and thus are particles that may lead to a decrease in the magneticrecording density of the medium. Therefore, it is preferred that thenumber ratio of particles with a TEM particle diameter of 20 nm or moreis also small. In the iron-based oxide magnetic powder of the presentinvention, the number ratio of particles with a TEM particle diameter of20 nm or more is 25% or less, and therefore, the magnetic recordingdensity when the iron-based oxide magnetic powder is used in a magneticrecording medium can be improved as compared with the case where aconventional iron-based oxide magnetic powder is used.

In the iron-based oxide magnetic powder of the present invention, withrespect to the ε-iron oxide particles or the substitution-type ε-ironoxide particles that constitute the magnetic powder, the coefficient ofvariation of the particle diameter is set to 19% or less. Setting thecoefficient of variation of the particle diameter to 19% or less worksadvantageously for the improvement of the electromagnetic conversionproperty when the iron-based oxide magnetic powder is used in a magneticrecording medium.

[Average Circularity]

In the present invention, the particle shape of the ε-iron oxideparticle or the substitution-type ε-iron oxide particle is evaluatedbased on the average circularity of the particles observed with atransmission electron microscope defined below. The circularity is aconcept that evaluates how close the shape of a certain particle is to aperfect sphere, and refers to a numerical value obtained by dividing theproduct of the projected area of the particle and 4π by the square ofthe peripheral length of the projected image of the particle and takes avalue from 0 to 1. In the case of a perfect circle, the circularityis 1. The method for measuring the circularity will be described later,but for the substitution-type ε-iron oxide particles, by increasing thenumber of particles for which the circularity is measured andcalculating the average value, the displacement from the perfect sphereof the ε-iron oxide particle or the substitution-type ε-iron oxideparticle can be evaluated.

It is preferred that in the ε-iron oxide particles or thesubstitution-type ε-iron oxide particles that constitute the iron-basedoxide magnetic powder of the present invention, the average circularityof particles observed with a transmission electron microscope is 0.955or more. When the average circularity of the particles is 0.955 or more,the substitution-type ε-iron oxide particles are easily oriented when amagnetic recording medium is produced by orienting the ε-iron oxideparticles or the substitution-type ε-iron oxide particles by an externalmagnetic field, and therefore, as a result, the magnetic recordingdensity of the magnetic recording medium can be improved. The averagecircularity of particles observed with a transmission electronmicroscope is more preferably 0.96 or more.

[Method for Producing Iron-Based Oxide Magnetic Powder]

The method for producing an iron-based oxide magnetic powder of thepresent invention is characterized in that precursor particles thatserve as a precursor of the ε-iron oxide particles or thesubstitution-type ε-iron oxide particles have a larger particle diameterand better dispersibility than precursor particles obtained by aconventional known production method. The present inventors presumedthat in the step of producing precursor particles disclosed in PTL 4,very fine precursor particles with a particle diameter of 1 nm or lessare produced, and the precursor particles are coated with a siliconcompound in a state of being aggregated to a size on the order ofseveral tens of nanometers, and then subjected to a firing step, and asa result, iron-based oxide particles with an average particle diameterof about 10 to 30 nm are formed. In the production method disclosed inPTL 4, it is difficult to arbitrarily control the degree of aggregationof the precursor particles, and it is considered that as a result, onlyan iron-based oxide magnetic powder with a wide particle diameterdistribution is obtained. In the production method of the presentinvention, by improving the method for producing precursor particles,precursor particles having a large particle diameter and gooddispersibility are obtained, and it is considered that by using such aprecursor, the production ratio of particles finer than the averageparticle diameter or coarse iron-based oxide particles is reduced, andthe ε-iron oxide particles or the substitution-type ε-iron oxideparticles having a particle shape closer to a perfect sphere areobtained.

Further, in the method for producing an iron-based oxide magnetic powderof the present invention, the content of fine particles can be furtherreduced by classifying the slurry containing the ε-iron oxide particlesor the substitution-type ε-iron oxide particles having a particle shapecloser to a perfect sphere.

[Starting Material]

In the method for producing an iron-based oxide magnetic powder of thepresent invention, an acidic aqueous solution (raw material solution)containing trivalent iron ions, or trivalent iron ions and metal ions ofa metal element that finally substitutes Fe sites of €-iron oxideparticles is used as a starting material of the iron-based oxidemagnetic particle powder. Note that in the description, the “acidic”means that the pH is 7.0 or lower. As the supply source of the iron ionsor the metal ions of a substituent element, a water-soluble inorganicacid salt such as a nitrate, a sulfate, or a chloride can be used fromthe viewpoint of ease of availability and price. In order to finallyobtain the ε-iron oxide particles or the substitution-type ε-iron oxideparticles, it is preferred to pass through β-FeOOH as a precursor, and ahalogen ion is required for obtaining β-FeOOH as the crystal structureof the precursor. Therefore, it is preferred to use iron(III) chlorideamong the illustrated iron salts.

In the production method of the present invention, in the raw materialsolution preparation step, the above-mentioned raw material solution andan aqueous solution of an alkali used for neutralizing the raw materialsolution in the first neutralization step described later are preparedin advance. The alkali for neutralizing the raw material solution may beany of an alkali metal or alkaline earth hydroxide, ammonia water, andan ammonium salt such as ammonium hydrogen carbonate, but it ispreferred to use ammonia water or ammonium hydrogen carbonate with whichan impurity is less likely to remain when the ε-iron oxide particles orthe substitution-type ε-iron oxide particles are finally formed byperforming a heat treatment.

[First Neutralization Step]

The most important technical feature in the method for producing aniron-based oxide magnetic powder of the present invention is that in thefirst neutralization step, each of the raw material solution obtained inthe previous step and the alkaline aqueous solution for neutralizing theraw material solution is added to a reaction system. Note that thereaction system will be described later.

In a conventional method for producing an iron-based oxide magneticpowder composed of ε-iron oxide particles or substitution-type ε-ironoxide particles using an aqueous solution system, in general, analkaline aqueous solution is continuously added to the entire amount ofa raw material solution to be used in the reaction. In this case, a timelag occurs until the reaction system after adding the alkaline aqueoussolution becomes in a uniformly mixed state, and local fluctuationoccurs in the compositional ratio of metal ions and OH⁻ ions in thereaction system, and therefore, it is presumed that generation frequencyand particle diameter of the above-mentioned precursor generated by theneutralization reaction locally vary, and it is difficult to control thedegree of aggregation of precursor particles.

On the other hand, in the first neutralization step in the productionmethod of the present invention, the concentrations of the raw materialsolution and the alkaline aqueous solution to be added, and the additionrate thereof to the reaction system can be arbitrarily changed, andtherefore, it becomes possible to more uniformly control thecompositional ratio of metal ions and OH⁻ ions in the reaction system,and precursor particles having a large particle diameter and gooddispersibility are obtained, and as a result, an iron-based oxidemagnetic powder having a narrow particle diameter distribution isobtained.

In the first neutralization step in the production method of the presentinvention, the raw material solution obtained in the previous step andthe alkaline aqueous solution are continuously or intermittently addedto the reaction system and mixed so as to adjust the pH of the reactionsystem to 1.0 or higher and 3.0 or lower. It is considered that in theaqueous solution obtained by mixing the raw material solution and thealkaline aqueous solution, although the detailed crystal structure isunknown, very fine particles of an oxyhydroxide of the trivalent iron orthe substituent metal element, a hydroxide of the trivalent iron or thesubstituent metal element, or a mixture thereof are generated, and acolloidal solution is formed. In the production method of the presentinvention, the compositional ratio of metal ions and OH⁻ ions in thereaction system can be strictly controlled, and therefore, thegeneration state of such colloidal particles can be controlled to adesired state.

In the present specification, the reaction system refers to a reactionsolution in which the raw material solution and the alkaline aqueoussolution are mixed and a neutralization reaction occurs.

In a first embodiment of the present invention, the first neutralizationstep is typically started by simultaneously adding the raw materialsolution and the alkaline aqueous solution into pure water that is thereaction system which does not contain trivalent iron ions or ions of ametal element that partially substitutes the Fe sites. In this case, thereaction system at the initial stage is pure water, and the reactionsystem after the start of addition of the raw material solution and thealkaline aqueous solution becomes a reaction solution in which purewater, the raw material solution, and the alkaline aqueous solution aremixed.

In the first embodiment of the production method, the raw materialsolution and the alkaline aqueous solution may be directly andsimultaneously added to an empty reaction vessel for holding thereaction solution. In this case, the reaction system is a mixed solutionof the raw material solution and the alkaline aqueous solution.

In a second embodiment of the production method of the presentinvention, the reaction system at the initial stage is an acidic metalsalt solution containing trivalent iron ions, or trivalent iron ions andions of a metal element that partially substitutes the Fe sites, and theraw material solution and the alkali are simultaneously added into themetal salt solution. Here, the type of the metal salt contained in themetal salt solution may be the same as the mode of the metal saltcontained in the above-mentioned raw material solution. In this case,the amount of the metal ions contained in the metal salt solution thatis the reaction system at the initial stage is preferably set to 50 mol% or less with respect to the sum of the amount of the metal ionscontained in the metal salt solution and the amount of the metal ionscontained in the raw material solution added in the first neutralizationstep.

In the first neutralization step of the first embodiment of theproduction method of the present invention, the raw material solutionand the alkaline aqueous solution containing the alkali in an amount of0.4 equivalents or more and 0.9 equivalents or less with respect to thetotal amount of an acid group contained in the raw material solution areadded so as to adjust the pH of the reaction system to 1.0 or higher and3.0 or lower. Note that the acid group refers to Cl⁻ in the case of achloride, NO₃ ⁻ in the case of a nitrate, and SO₄ ²⁻ in the case of asulfate.

In this case, the equivalent can be represented as follows.

[moles of alkali used in neutralization×alkali valence]/[sum of (molesof acid group contained in raw material solution× acid group valence)]

Here, the alkali valence is the number of moles of OH-ionsstoichiometrically generated from 1 mol of the alkali, and for example,the alkali valences of NaOH, Ca(OH)₂, and NH₃ are 1, 2, and 1,respectively. Further, the acid group valence is the valence of an acidgroup anion, and for example, the acid group valences of a chloride Cl⁻,a nitrate NO₃ ⁻, and a sulfate SO₄ ²⁻ are 1, 1, and 2, respectively.

When the amount of the alkali to be added is less than 0.4 equivalents,the amount of colloids to be generated by neutralization of the rawmaterial solution becomes small and the effect of the present inventioncannot be obtained, and therefore, it is not preferred. When the amountof the alkali to be added exceeds 0.9 equivalents, the dispersibility ofthe colloidal solution to be obtained deteriorates and the effect of thepresent invention cannot be obtained, and therefore, it is notpreferred. When the pH after neutralization is lower than 1.0, theamount of colloids to be generated by neutralization of the raw materialsolution becomes small and the effect of the present invention cannot beobtained, and therefore, it is not preferred. When the pH afterneutralization exceeds 3.0, the dispersibility of the colloidal solutionto be obtained deteriorates and the effect of the present inventioncannot be obtained, and therefore, it is also not preferred.

Note that the value of pH described in the present description wasmeasured using a glass electrode according to JIS Z 8802. The pHstandard solution refers to a value measured with a pH meter calibratedusing a suitable buffer solution corresponding to the pH range to bemeasured. Further, the pH described in the present description is avalue obtained by directly reading a measured value indicated by a pHmeter compensated with a temperature compensation electrode under thereaction temperature conditions.

The addition mode of the raw material solution and the alkaline aqueoussolution may be continuous or intermittent. Further, the addition rateof each of the raw material solution and the alkaline aqueous solutionmay be constant or may be changed in the middle of addition. Withrespect to the timing of addition of the raw material solution and thealkaline aqueous solution, the addition starts at the same time, butwith respect to the timing of completion of addition, it does not matterif there is a time when the addition of either of the raw materialsolution and the alkaline aqueous solution is completed first and onlythe other is added.

In the first neutralization step of the production method of the presentinvention, the raw material solution and the alkaline aqueous solutionare continuously or intermittently added to the reaction system, andtherefore, it is preferred to control the addition rate of each usingthe concept of “cumulative alkali addition equivalent” which is theequivalent number of cumulative addition amount of the alkali withrespect to 1 equivalent of the total cumulative addition amount of theacid group contained in the raw material solution. The cumulative alkaliaddition equivalent can be represented as follows.

[Cumulative amount of moles of alkali used in neutralization×alkalivalence]/[sum of (cumulative amount of moles of acid group contained inraw material solution×acid group valence)]

Here, the cumulative amount of moles of alkali refers to the cumulativeamount of the number of moles of the alkali added, and the cumulativeamount of moles of acid group refers to the cumulative amount of thenumber of moles of the acid group added.

In the first neutralization step of the first embodiment of theproduction method of the present invention, it is preferred to adjustthe addition rate of each of the raw material solution and the alkalineaqueous solution so that the cumulative alkali addition equivalent ismaintained within a range of 0.4 equivalents or more and 0.9 equivalentsor less through the first neutralization step. It is more preferred toset the cumulative alkali addition equivalent to 0.6 equivalents or moreand 0.9 equivalents or less through the first neutralization step.

Strictly speaking, if the addition start times of the raw materialsolution and the alkaline aqueous solution do not exactly match, thecumulative alkali addition equivalent may not become 0.4 equivalents ormore and 0.9 equivalents or less at the very early stage of the start ofthe reaction in some cases, however, in this case, the addition starttimes of the raw material solution and the alkaline aqueous solution areregarded as completely the same.

By adjusting the cumulative alkali addition equivalent in the firstneutralization step to 0.4 equivalents or more through the firstneutralization step, an increase in the concentration of free trivalentiron ions in the reaction system can be suppressed, and therefore, thenumber of generated precursor nuclear particles to be formed when thealkali is added to the reaction system can be reduced. As a result, theparticle diameter of the colloid contained in the colloidal solutionobtained in the first neutralization step can be made larger than in thecase of PTL 4, and the particle diameter of the precursor particles ofthe ε-iron oxide particles or the substitution-type ε-iron oxideparticles obtained in the second neutralization step which is thesubsequent step can be made larger. If there is a time when thecumulative alkali addition equivalent is not maintained at 0.4equivalents or more and becomes less than 0.4 equivalents, the particlediameter of the colloid contained in the colloidal solution obtained inthe first neutralization step becomes smaller, and as a result, theeffect of the present invention sometimes cannot be obtained.

By adjusting the cumulative alkali addition equivalent in the firstneutralization step to 0.9 equivalents or less through the firstneutralization step, the pH of the reaction system can be prevented fromexceeding 3.0, and the grains of the precursor particles can grow in adispersed state in the reaction system. As a result, it is possible toobtain precursor particles having good dispersibility (that is, causingless interparticle aggregation) and a narrow particle size distributionin the second neutralization step that is the subsequent step. If thereis a time when the cumulative alkali addition equivalent is notmaintained at 0.9 equivalents or less and exceeds 0.9 equivalents, thedispersibility of the colloidal solution obtained in the firstneutralization step deteriorates (that is, interparticle aggregationincreases), and as a result, the effect of the present invention cannotbe obtained in some case.

In the first neutralization step of the second embodiment of theproduction method of the present invention, the raw material solutionand the alkaline aqueous solution containing the alkali in an amount of0.4 equivalents or more and 1.8 equivalents or less with respect to thetotal amount of the acid group contained in the raw material solutionare added so as to adjust the pH of the reaction system to 1.0 or higherand 3.0 or lower. In addition, it is preferred that in the firstneutralization step of the second embodiment of the production method ofthe present invention, the addition rate of each of the raw materialsolution and the alkaline aqueous solution is adjusted so that thecumulative alkali addition equivalent is maintained within a range of0.4 equivalents or more and 1.8 equivalents or less and the pH of thereaction system is maintained within a range of 3.0 or lower (preferably0.0 or higher) through the neutralization step, and the pH at the end ofthe first neutralization step becomes 1.0 or higher and 3.0 or lower.

The total metal ion concentration in the raw material solution is notparticularly specified in the present invention, but it is preferred toadjust the total concentration after alkali addition in the secondneutralization step to 0.01 mol/kg or more and 0.5 mol/kg or less. Whenit is less than 0.01 mol/kg, the amount of the iron-based oxide magneticpowder obtained by one reaction is small, which is economically notpreferred. When the total metal ion concentration exceeds 0.5 mol/kg,the reaction solution tends to gel due to the rapid occurrence ofprecipitation of a hydroxide, and therefore, it is not preferred.

The alkali concentration of the alkaline aqueous solution, in otherwords, the pH of the alkaline aqueous solution is also appropriatelyadjusted based on the same idea.

In the first neutralization step of the production method of the presentinvention, the raw material solution and the alkaline aqueous solutionare added to the reaction system over a period of 10 minutes or more andmixed. If the addition time is less than 10 minutes, the nuclearparticles of the precursor are generated at a time immediately after theaddition, and the ions that should have been used for nuclear growth arealso consumed, so that the size of the precursor becomes small, which isnot preferred from the viewpoint of obtaining the effect of the presentinvention.

Further, by extending the addition time, the particle diameter of thecolloid obtained in the first neutralization step can be controlled, butif it is extended too much, the productivity deteriorates, andtherefore, it is not preferred. Accordingly, the addition time ispreferably 10 minutes or more and 480 minutes or less, and morepreferably 60 minutes or more and 300 minutes or less.

In the production method of the present invention, the treatmenttemperature during the first neutralization treatment is notparticularly limited, but is preferably set to 20° C. or higher and 60°C. or lower.

[Hydroxycarboxylic Acid Addition Step]

In the production method of the present invention, a hydroxycarboxylicacid is added to the colloidal solution obtained in the firstneutralization step described above. The hydroxycarboxylic acid is acarboxylic acid having an OH group in the molecule and acts as acomplexing agent for an iron ion. Here, it is considered that thehydroxycarboxylic acid has an effect of forming a complex with atrivalent iron ion dissolved in the reaction solution, and delaying ahydroxide formation reaction of iron when the second neutralizationtreatment is performed in the subsequent step, resulting in narrowingthe distribution of the average particle diameter of the precursor fineparticles containing an iron oxyhydroxide to be produced. Note that itis considered that the hydroxycarboxylic acid also forms a partialcomplex with the metal ion of the substituent element. The type and theaddition amount of the hydroxycarboxylic acid may be in accordance witha known technique of PTL 4.

There exist many types of hydroxycarboxylic acids such as glycolic acid,lactic acid, various hydroxybutyric acids, glyceric acid, malic acid,tartaric acid, citric acid, and mevalonic acid, but a polyvalentaliphatic hydroxycarboxylic acid is preferred from the viewpoint of acomplexing ability, and tartaric acid, citric acid, or malic acid ismore preferred in terms of price and ease of availability.

Further, with respect to the addition amount of the hydroxycarboxylicacid, the addition amount of the hydroxycarboxylic acid is preferablysuch that the molar ratio thereof with respect to the total amount ofthe trivalent iron ions and the ions of the substituent metal elementM(D/(Fe+M)) is 0.125 or more and 1.0 or less.

The hydroxycarboxylic acid may be added in a mechanically stirred statewithout particularly changing the reaction temperature in the firstneutralization step, which is the previous step. Although it may beadded as a solid to the reaction solution, it is preferably added in anaqueous solution state from the viewpoint of ensuring the uniformity ofthe reaction.

[Second Neutralization Step]

In the production method of the present invention, an alkali is furtheradded to the reaction solution after the addition of thehydroxycarboxylic acid so as to neutralize the pH to 7.0 or higher and10.0 or lower. As the alkali to be added, the alkaline aqueous solutionused in the first neutralization step described above can be used, but asolution in which the alkali concentration is changed may be used. Bythis step, the nuclei of the iron oxyhydroxide, which is the precursorof the substitution-type ε-iron oxide particles produced in the firstneutralization step, grow and final precursor particles can be formed.

In this step, by setting the pH after neutralization to 7.0 or higherand 10.0 or lower, when the trivalent iron and the substituent metalelement are neutralized, the iron oxyhydroxide containing thesubstituent metal element, which is a solid material, can be obtainedwithout leaving dissolved ions of the trivalent iron and the substituentmetal element in the reaction solution. If the pH after neutralizationis set lower than 7.0, the substituent metal element which is notcompletely neutralized in the first neutralization step, for example, Cois left as an ion in the liquid to cause composition deviation in thesolid material to be collected, and also waste the substituent metalelement, and therefore, it is not preferred also from the viewpoint ofproduction cost. Further, when the pH after neutralization exceeds 10.0,the effect of neutralization is saturated, and when, for example, anamphoteric metal such as Al is used as the substituent metal element,there arise problems that an ion is left in the liquid, the compositiondeviates, and the substituent metal element is wasted in the same manneras in the case where the pH is lower than 7.0, and therefore, it is notpreferred.

The particles of the iron oxyhydroxide or the particles of thesubstituent metal element-containing iron oxyhydroxide (precursorparticles) contained in the slurry obtained in this step have an averagediameter measured with a dynamic light scattering particle sizedistribution analyzer (hereinafter sometimes referred to as a DLSaverage diameter) of preferably 300 nm or less, more preferably 100 nmor less, and further more preferably 70 nm or less. The small DLSaverage diameter in this manner means that the dispersibility of theprecursor particles is good and the degree of aggregation is low, and isadvantageous to the s-iron oxide particles or the substitution-typeε-iron oxide particles that constitute the iron-based oxide magneticpowder to be finally obtained for reducing the number ratio of coarseparticles with a particle diameter of 20 nm or more or the like. Thelower limit of the DLS average diameter of the slurry containing theprecursor particles is not particularly limited, but one having adiameter of about 20 nm or more is obtained by the production method ofthe present invention.

[Water Washing Step]

In the production method of the present invention, the iron oxyhydroxideas the precursor produced in the steps up to this point is small also interms of DLS average diameter and has high dispersibility. However, theionic strength in the solution increases as it goes through thehydroxycarboxylic acid addition step and the neutralization step in thesteps up to the previous stage. If the ionic strength remains high, itwill become an aggregation system in the subsequent coating step withsilicon oxide, and therefore, it is not preferred. Therefore, it ispreferred to wash the slurry obtained in the above step with water toreduce the ionic strength in the solution and bring it in a dispersedstate. The water washing method is not particularly specified, but amethod of performing a water washing treatment in a slurry state as suchis preferred in consideration of maintenance of particle dispersibilityin this step, washing uniformity, connection with the steps before andafter this step, handleability, and the like. When considering this,water washing using an ultrafiltration membrane or an ion exchangemembrane is preferred. In the case of washing using an ultrafiltrationmembrane, as the membrane, one having a molecular weight cut-off throughwhich particles do not pass to the filtrate side is used, and withrespect to completion of the washing, it is preferred to perform thewashing until the electrical conductivity of the filtrate becomes 200mS/m or less, more preferably 100 mS/m or less, and further morepreferably 30 mS/m or less. When there are many residual ions, themelting point of silicon oxide is locally lowered in the subsequentheating step, and therefore, there is a problem that coarse particlesare easily generated by sintering when obtaining ε-iron oxide.

[Coating Step with Silicon Oxide]

In the production method of the present invention, the precursorparticles formed in the steps up to this point hardly undergo phasetransition to the substitution-type ε-iron oxide particles even if theparticles are subjected to a heat treatment as they are, and therefore,prior to the heat treatment, a silicon oxide coating is applied to theprecursor particles. As a coating method with silicon oxide, it ispreferred to apply a sol-gel method. Note that the silicon oxide hereincludes not only one having a stoichiometric composition, but also onehaving a nonstoichiometric composition such as the below-mentionedsilanol derivative or the like.

In the case of the sol-gel method, to an aqueous solution of ironoxyhydroxide crystals or iron oxyhydroxide crystals containing asubstituent element dispersed after the precursor reaction, a siliconcompound having a hydrolyzable group, for example, a silane compoundsuch as tetraethoxysilane (TEOS), tetramethoxysilane (TMOS), or any ofvarious types of silane coupling agents is added to cause a hydrolysisreaction under stirring, and the surface of the iron oxyhydroxidecrystal is coated with the generated silanol derivative. Further, anacid catalyst or an alkali catalyst may be added thereto. It ispreferred to add it when considering the treatment time. Representativeexamples thereof include hydrochloric acid as the acid catalyst andammonia as the alkali catalyst. When the acid catalyst is used, it isnecessary to limit the addition of the acid catalyst to such an amountthat the iron oxyhydroxide particles or the iron oxyhydroxide particlescontaining a substituent element are not dissolved.

Note that as a specific method for coating with silicon oxide, the samemethod as the sol-gel method in a known process can be adopted. Forexample, the reaction temperature for the silicon oxide coating in thesol-gel method is 20° C. or higher and 60° C. or lower, and the reactiontime is about 1 hour or more and 20 hours or less. After the coatingtreatment with silicon oxide, solid-liquid separation and a dryingtreatment are performed to prepare a sample before the heating step.Here, after the coating treatment, water washing may be performed beforesolid-liquid separation. Further, here, at the time of solid-liquidseparation, a flocculation agent may be added to perform thesolid-liquid separation.

[Heating Step]

In the production method of the present invention, the ε-iron oxideparticles or the substitution-type ε-iron oxide particles are obtainedby subjecting the precursor particles coated with silicon oxide to aheating treatment. Prior to the heating treatment, washing and dryingsteps may be provided. The heating treatment is performed in anoxidative atmosphere, and the oxidative atmosphere may be an airatmosphere. The heating treatment temperature varies depending on thecoating amount of silicon oxide and therefore cannot be generalized, butthe heating can be generally performed within a range of 700° C. orhigher and 1300° C. or lower. When the heating temperature is too low, adifferent phase or a compound whose phase transition is not sufficientis likely to be mixed. When the lower limit of the heating temperatureis set to 700° C. or higher, the substitution-type ε-iron oxideparticles that are the target of the present invention can beselectively and stably obtained, and as a result, the generation of adifferent phase or a compound whose phase transition is not sufficientcan be suppressed. When the heating temperature is high, α-Fe₂O₃ that isa thermodynamically stable phase or partially substituted α-Fe₂O₃ islikely to be generated, and therefore, the heating treatment isperformed preferably at 900° C. or higher and 1200° C. or lower, andmore preferably at 950° C. or higher and 1150° C. or lower. The heatingtreatment time can be adjusted within a range of about 0.5 hours or moreand 10 hours or less, but favorable results are easily obtained within arange of 2 hours or more and 5 hours or less.

[Silicon Oxide Coating Removal Step]

By the above steps, the ε-iron oxide particles or the substitution-typeε-iron oxide particles coated with silicon oxide are obtained, but inthe production method of the present invention, prior to thebelow-mentioned classification treatment, the silicon oxide coating isdissolved and removed.

As a specific method, since silicon oxide is soluble in an alkalineaqueous solution, silicon oxide can be dissolved and removed byimmersing the ε-iron oxide particles or the substitution-type ε-ironoxide particles coated with silicon oxide after the heating treatment inan aqueous solution in which a strong alkali such as NaOH or KOH isdissolved and stirring the aqueous solution. When the dissolution rateis increased, the alkali aqueous solution may be heated.Representatively, when an alkali such as NaOH is added in an amount notless than 3 molar times the amount of silicon oxide, and the slurrycontaining the powder is stirred in a state where the temperature of theaqueous solution is 60° C. or higher and 70° C. or lower, silicon oxidecan be favorably dissolved.

[Iron Oxide Particle-Containing Slurry]

In the production method of the present invention, the ε-iron oxideparticles or the substitution-type ε-iron oxide particles obtained bythe silicon oxide coating removal step described above are dispersed inwater to form a slurry. In this case, it is preferred to perform washingusing pure water until the electrical conductivity of the solventbecomes 15 mS/m or less from the viewpoint of suppression of aggregationof the iron oxide particles and reduction of impurities. Thereafter, thepH of the slurry is adjusted to 11.0 or higher and 14.0 or lower byadding a quaternary ammonium salt to the slurry. By adding a quaternaryammonium salt to the slurry to adjust the pH of the slurry to 11.0 orhigher and 14.0 or lower, excellent dispersibility of thesubstitution-type ε-iron oxide particles in the slurry is ensured. Whenthe pH of the slurry is lower than 11.0, the dispersibility of theε-iron oxide particles or the substitution-type ε-iron oxide particlesdeteriorates, and also when the pH of the slurry exceeds 14.0, theeffect of improving the dispersibility is saturated and the chemicalcost of the alkali increases, and therefore, each case is not preferred.Note that as will be described later, when a hydroxide of atetraalkylammonium salt is used as the quaternary ammonium salt, the pHof the slurry falls within the above-mentioned range without addinganother alkali.

[Quaternary Ammonium Ion]

The quaternary ammonium ion is a cation resulting from substitution ofall four hydrogen atoms of an ammonium ion (NH₄ ⁺) by an organic group,and as the substituent, an alkyl group or an aryl group is generallyused. In the production method of the present invention, as a surfacemodifier for the ε-iron oxide particles or the substitution-type ε-ironoxide particles, a quaternary ammonium ion that is stable in a strongalkaline region in which the dispersibility of the iron oxide particlesis good is used. Among the quaternary ammonium ions, it is preferred touse a tetraalkylammonium ion that is industrially easily available. Thetetraalkylammonium ion is a quaternary ammonium cation and is apolyatomic ion whose molecular formula is represented by NR₄ ⁺ (R is anarbitrary alkyl group). The tetraalkylammonium salt is a completelydissociated salt and is present as an ion stable in an alkaline aqueoussolution, and therefore is used as a surface modifier for improving thedispersibility of the substitution-type ε-iron oxide particles in theslurry in the production method of the present invention. Examples of asupply source of the tetraalkylammonium ion include a hydroxide, achloride, and a bromide, but a hydroxide of the tetraalkylammonium ionis a strong alkali by itself, and therefore, when it is added to theslurry, the pH of the slurry falls within the above-mentioned preferredpH range without adding another alkali, and thus, it is more preferredto use tetraalkylammonium hydroxide as the surface modifier.

As the supply source of the tetraalkylammonium ion, a chloride or abromide of a tetraalkylammonium ion can also be used, but in this case,an increase in pH is suppressed when such a salt is added to the slurry,and an additional alkali is needed for adjusting the pH, so that the ionstrength of the system increases, and therefore, the dispersibility ofthe substitution-type ε-iron oxide particles is slightly inferior to thecase where a hydroxide is used.

Examples of the tetraalkylammonium salt include quaternary ammoniumsalts having the same alkyl group such as a tetramethylammonium salt, atetrapropylammonium salt, and a tetrabutylammonium salt, and quaternaryammonium salts having different alkyl groups, and any of them can beused, but it is preferred to use a hydroxide such as tetramethylammoniumhydroxide, tetrapropylammonium hydroxide, and tetrabutylammoniumhydroxide, respectively.

The concentration of the tetraalkylammonium ion is set to 0.009 mol/kgor more and 1.0 mol/kg or less in order to modify the surfaces of theε-iron oxide particles or the substitution-type ε-iron oxide particlesin the slurry and enhance the dispersibility thereof. In either casewhere the concentration is less than 0.009 mol/kg or more than 1.0mol/kg, the effect of improving the dispersibility is not sufficient,and one having a CV value of 19% or less cannot be obtained afterclassification.

[Classification Treatment]

In the production method of the present invention, the surfacemodifier-containing slurry obtained by adding the surface modifier issubjected to a dispersion treatment, whereby an iron oxide particledispersion slurry is obtained. Here, the dispersion treatment is atreatment of loosening the aggregation of aggregates of the ε-iron oxideparticles or the substitution-type ε-iron oxide particles contained inthe surface modifier-containing slurry. As the dispersion treatmentmethod, a known method such as dispersion with an ultrasonic disperser,pulverization using a medium such as a bead mill, or stirring with astirring blade, a shaking machine, or a shaker can be adopted.

The dispersion treatment of the surface modifier-containing slurry ispreferably performed until the average secondary particle diameter (DLSparticle diameter) of the iron oxide particles measured with a dynamiclight scattering particle size distribution analyzer becomes 65 nm orless. In a state where the DLS particle diameter exceeds 65 nm, when theparticle diameter is small, such as a TEM average particle diameter of20 nm or less, one having a CV value of 19% or less cannot be obtained,and therefore, such a state is not preferred.

[Classification Operation]

In the production method of the present invention, the surfacemodifier-containing slurry obtained by adding the surface modifier issubjected to the dispersion treatment, and the ε-iron oxide particle orsubstitution-type ε-iron oxide particle dispersion slurry is subjectedto a classification operation. As the classification operation, a knownwet classification method such as a centrifugation method can beadopted. Fine particles can be removed by removing the supernatant afterthe slurry is subjected to a centrifuge, and coarse particles can beremoved by removing the precipitate after the slurry is subjected to acentrifuge. The gravitational acceleration during centrifugation ispreferably 40,000 G or more. Further, it is preferred to repeat theclassification operation three or more times. After the classificationoperation, the magnetic powder composed of the ε-iron oxide particles orthe substitution-type ε-iron oxide particles is collected using a knownsolid-liquid separation method, and washed with water as needed, andthen dried.

[Transmission Electron Microscope (TEM) Observation]

TEM observation of the magnetic powder composed of the substitution-typeε-iron oxide obtained by the production method of the present inventionwas performed under the following conditions. In the TEM observation,JEM-1011 manufactured by JEOL Ltd. was used. For particle observation,TEM photographs taken at a magnification of 10,000 times and amagnification of 100,000 times were used (those after removing thesilicon oxide coating were used).

—Measurement of Average Particle Diameter, Particle Size Distribution,and Circularity—

Digitization was used in the evaluation of the TEM average particlediameter, particle size distribution, and circularity. Mac-View Ver. 4.0was used as the image processing software. When the image processingsoftware is used, the particle diameter of a certain particle iscalculated as the length of the long side of a rectangle having thesmallest area among the rectangles circumscribing the particle. Further,the circularity of a certain particle is calculated as a value obtainedby dividing the product of the area of the particle and 4π by the squareof the peripheral length of the particle. As for the number ofparticles, 200 or more particles were measured.

The selection criteria for the particles to be measured among theparticles shown in the transmission electron micrograph were as follows.

[1] A particle that is partially outside the visual field of themicrograph is not measured.

[2] A particle with a well-defined contour and existing independently ismeasured.

[3] Even if the particle shape deviates from the average particle shape,a particle that is independent and can be measured as a single particleis measured.

[4] Particles that overlap each other, but the boundaries between theparticles are clear, and also the shape of the entire particle can bedetermined are each measured as a single particle.

[5] Overlapping particles whose boundaries are not clear and also whoseoverall shape is unknown are regarded as particles whose shape cannot bedetermined and are not measured.

The number average particle diameter of the particles selected based onthe above criteria was calculated and defined as the average particlediameter by TEM observation of the iron oxide magnetic powder. Further,a value obtained by dividing the “standard deviation of the particlediameter of the selected particle” by the “number average particlediameter (=average particle diameter) of the selected particles” wascalculated and defined as the coefficient of variation of the particlediameter by TEM observation of the iron oxide magnetic powder. Inaddition, the number average circularity of the selected particles wascalculated and defined as the average circularity by TEM observation ofthe iron oxide magnetic powder.

[DLS Average Diameter]

The DLS particle diameter by a dynamic light scattering particle sizedistribution analysis of the slurry containing the iron oxyhydroxide orthe substituent metal element-containing iron oxyhydroxide obtained bythe production method of the present invention, and the iron oxideparticle dispersion slurry after the dispersion treatment was measuredunder the following conditions.

As the dynamic light scattering particle size distribution analyzer,(FPAR-1000K high sensitivity specification) manufactured by OtsukaElectronics Co., Ltd. was used, and as a fiber probe, a dilution-typeprobe was used. The measurement was performed under the followingmeasurement conditions: a measurement time: 180 seconds, the number ofrepetitions: 1, and solvent setting: water. As the analysis mode, theCumulant method was used.

[Compositional Analysis by High-Frequency Inductively Coupled PlasmaAtomic Emission Spectroscopy (ICP)]

The compositional analysis of the obtained magnetic powder composed ofthe substitution-type ε-iron oxide particles was performed. In thecompositional analysis, ICP-720ES manufactured by Agilent Technologies,Inc. was used, and the analysis was performed by setting the measurementwavelength (nm) as follows: Fe: 259.940 nm, Ga: 294.363 nm, Co: 230.786nm, Ti: 336.122 nm, Ni: 231.604 nm, and Cr: 267.716 nm. As themeasurement wavelength of each metal element, a wavelength at which thelinearity of the calibration curve can be obtained without interferencewith the spectra of other elements is selected according to thecomposition of the iron-based oxide magnetic powder to be analyzed.

[Measurement of Magnetic Hysteresis Curve (B-H Curve)]

The magnetic properties of the obtained magnetic powder composed of thesubstitution-type ε-iron oxide particles were measured under thefollowing conditions.

In the case of the iron-based oxide magnetic powder composed of thesubstitution-type ε-iron oxide particles, as a magnetic propertymeasuring device, a vibrating sample magnetometer (VSM) (VSM-5,manufactured by Toei Industry Co., Ltd.) was used, and magneticproperties were measured at an applied magnetic field of 1035 kA/m (13kOe), an M measurement range of 0.005 A·m² (5 emu), an applied magneticfield change rate of 15 (kA/ms), a time constant of 0.03 sec, and a waittime of 0.1 sec. Evaluation was performed for the coercive force Hc,saturation magnetization os, SFD, and square ratio SQ based on a B-Hcurve, and also evaluation for a low Hc component that does notcontribute to magnetic recording was performed based on a differentialB-H curve. Further, in this case, the intercept of the vertical axis inthe 0 magnetic field of the differential B-H curve was representedI_(L), and the peak height on the high Hc side was represented I_(H).Note that the SFD refers to a value obtained by dividing the half widthof the peak on the high Hc side by the coercive force Hc. In addition,in the measurement and evaluation, the attached software (Ver. 2.1)manufactured by Toei Industry Co., Ltd. was used. Further, the squareratio SQ is one for the entire B-H curve. When the value of thedifferential B-H curve at the maximum magnetic field was represented byI_(Hmax), in the case where I_(Hmax)/I_(H) was 0.2 or less, the abovemeasurement results were used. When I_(Hmax)/I_(H) exceeded 0.2,remeasurement was performed according to the following procedure.

As a result of measurement at an applied magnetic field of 1035 kA/m (13kOe), when I_(Hmax)/I_(H) exceeded 0.2, a high temperaturesuperconducting type VSM (VSM-5HSC, manufactured by Toei Industry Co.,Ltd.) was used, and magnetic properties were measured at an appliedmagnetic field Hmax of 2387 kA/m (30 kOe), an M measurement range of0.005 A·m² (5 emu), an applied magnetic field change rate of 30kA/(m·s), a time constant of 0.03 sec, and a wait time of 0.1 sec. As aresult, when I_(Hmax)/I_(H) was 0.2 or less, the above measurementresults were used, and when I_(Hmax)/I_(H) exceeded 0.2, remeasurementwas performed according to the following procedure.

As a result of measurement at an applied magnetic field of 1035 kA/m (13kOe) and 2387 kA/m (30 kOe), when I_(Hmax)/I_(H) exceeded 0.2, a hightemperature superconducting type VSM (VSM-5HSC, manufactured by ToeiIndustry Co., Ltd.) was used, and magnetic properties were measured atan applied magnetic field Hmax of 3979 kA/m (50 kOe), an M measurementrange of 0.005 A·m² (5 emu), an applied magnetic field change rate of 30kA/(m·s), a time constant of 0.03 sec, and a wait time of 0.1 sec. As aresult, it was confirmed that I_(Hmax)/I_(H) is 0.2 or less, andtherefore, the measurement results were used.

EXAMPLES Example 1

As an iron raw material containing chloride ions, 349.8 g of ferric(III)chloride hexahydrate with a purity of 99 mass %, as raw materials ofsubstituent elements, 84.1 g of a gallium(III) nitrate solution with aGa concentration of 10.9 mass % and a nitrate concentration of 33 mass%, 15.8 g of cobalt(II) nitrate hexahydrate with a purity of 97 mass %,and 10.9 g of a titanium(IV) chloride solution with a Ti concentrationof 16.5 mass % and a chloride ion concentration of 31 mass %, and 468.9g of pure water were mixed, whereby a raw material aqueous solution wasprepared. The molar ratio of metal ions in the raw material aqueoussolution is as follows: Fe:Ga:Co:Ti=1.705:0.175:0.070:0.050.Subsequently, in a 5 L reaction vessel, 1500 g of pure water was placed,and the liquid temperature was adjusted to 40° C. while mechanicallystirring with a stirring blade. Subsequently, while continuing stirringand keeping the liquid temperature at 40° C., the raw material aqueoussolution and a 6.2% ammonia aqueous solution (as an alkali) weresimultaneously added into the pure water, and thereafter, stirring wasperformed for 60 minutes while keeping the liquid temperature at 40° C.,whereby a colloidal solution was obtained (procedure 1, firstneutralization step). Here, the raw material aqueous solution wascontinuously added over 240 minutes at an addition rate of 3.87 g/min,and the ammonia aqueous solution was continuously added over 240 minutesat an addition rate of 3.87 g/min. While adding the raw material aqueoussolution and the ammonia aqueous solution, the ratio of the cumulativeaddition amount of the alkali to the total cumulative addition amount ofthe acid group contained in the raw material solution was 0.75equivalents. The pH of the obtained colloidal solution was 1.68.

Subsequently, to the colloidal solution obtained by the procedure 1,288.8 g of a citric acid solution with a citric acid concentration of 20mass % as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 186.6 g of a 22.14% ammonia solution was addedthereto all at once, and then, the resulting mixture was held for 10minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding186.6 g of the ammonia solution all at once was 8.5.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 36.2 nm.

Subsequently, 935.7 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 1100 g. The temperature was adjusted to 40° C. while stirring, and1288.4 g of ethanol was added thereto, followed by stirring for 5minutes, and thereafter, 283.6 g of a 22.14 mass % ammonia aqueoussolution was added thereto. Further, 1130.3 g of tetraethoxysilane(TEOS) as a silicon compound having a hydrolyzable group was addedthereto over 35 minutes. Thereafter, while keeping the liquidtemperature at 40° C., stirring was continued as such for 1 hour tocarry out coating with a hydrolysate of the silicon compound.Thereafter, the obtained solution was washed and subjected tosolid-liquid separation, and the product was collected as a cake(procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1130° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 24 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 25 mass % tetramethylammoniumhydroxide (hereinafter referred to as TMAOH) aqueous solution was addedas a surface modifier, whereby a surface modifier-containing slurry witha solid content of 4.1% was obtained. Here, the addition amount of theTMAOH aqueous solution was set so that the TMAOH concentration in thesurface modifier-containing slurry became 0.065 mol/kg. In this case,the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of14,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 24,000 G. For a slurry on theprecipitate side, the operation of addition of 30 g of a 0.065 mol/kgTMAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 14,000 rpm and 24,000 G for 30 minutes, and collection of30 g of a supernatant slurry was repeated further 9 times, whereby atotal of 300 g of the supernatant slurry was collected. After 300 g ofthe collected supernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Kaki Co., Ltd.), and 30 g of a supernatant slurry containingfine particles was removed and a slurry on the precipitate side wasobtained. The gravitational acceleration in the centrifugation treatmentwas set to 39,000 G. To a slurry on the precipitate side obtained byrepeating the operation of addition of 30 g of a 0.065 mol/kg TMAOHaqueous solution, ultrasonic dispersion, a centrifugation treatment at18,000 rpm and 39,000 G for 30 minutes, and removal of 30 g of asupernatant slurry further 9 times for the obtained slurry on theprecipitate side, a 1 mass % sulfuric acid aqueous solution was added toadjust the pH to 6.5 and aggregate substitution-type ε-iron oxideparticles, followed by filtration through a membrane filter, and thesolid was collected and then dried, whereby an iron-based oxide magneticpowder according to Example 1 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of314 (kA/m), a saturation magnetization of 16.6 (Am²/kg), a square ratioof 0.637, an SFD of 0.71, and an I_(L)/I_(H) value of 0.14. Further, theobtained iron-based oxide magnetic powder was dispersed in pure waterusing an ultrasonic disperser to prepare a slurry, and the preparedslurry was dropped onto a collodion film on a grid and adhered thereto,and then, naturally dried. Thereafter, carbon vapor deposition wasperformed, and the resultant was subjected to TEM observation. As aresult of measuring 200 particles by TEM observation, the TEM averageparticle diameter was 16.0 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.0%, the number ratio ofparticles with a particle diameter of 20 nm or more was 1.3%, thecoefficient of variation (CV value) was 12%, and the average circularityof the particles was 0.974. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol. The above production conditions areshown in Tables 1 and 2, and the measurement results for the obtainediron-based oxide magnetic powder are shown in Tables 3 and 4 (the sameapplies to the following Examples and Comparative Examples).

Example 2

As an iron raw material containing chloride ions, 350.8 g of ferric(III)chloride hexahydrate with a purity of 99 mass %, as raw materials ofsubstituent elements, 95.3 g of a gallium(III) nitrate solution with aGa concentration of 11.0 mass % and a nitrate concentration of 33 mass,10.1 g of cobalt(II) nitrate hexahydrate with a purity of 97 mass, and9.8 g of a titanium(IV) chloride solution with a Ti concentration of16.5 mass % and a chloride ion concentration of 31 mass %, and 474.5 gof pure water were mixed, whereby a raw material aqueous solution wasprepared. The molar ratio of metal ions in the raw material aqueoussolution is as follows: Fe:Ga:Co:Ti=1.710:0.200:0.045:0.045.Subsequently, in a 5 L reaction vessel, 1500 g of pure water was placed,and the liquid temperature was adjusted to 40° C. while mechanicallystirring with a stirring blade. Subsequently, while continuing stirringand keeping the liquid temperature at 40° C., the raw material aqueoussolution and a 6.5% ammonia aqueous solution (as an alkali) weresimultaneously added into the pure water, and thereafter, stirring wasperformed for 60 minutes while keeping the liquid temperature at 40° C.,whereby a colloidal solution was obtained (procedure 1, firstneutralization step). Here, the raw material aqueous solution wascontinuously added over 240 minutes at an addition rate of 3.92 g/min,and the ammonia aqueous solution was continuously added over 240 minutesat an addition rate of 3.92 g/min. While adding the raw material aqueoussolution and the ammonia aqueous solution, the ratio of the cumulativeaddition amount of the alkali to the total cumulative addition amount ofthe acid group contained in the raw material solution was 0.80equivalents. The pH of the obtained colloidal solution was 1.85.

Subsequently, to the colloidal solution obtained by the procedure 1,288.8 g of a citric acid solution with a citric acid concentration of 20mass % as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 189.8 g of a 21.87% ammonia solution was addedthereto all at once, and then, the resulting mixture was held for 10minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding189.8 g of the ammonia solution all at once was 8.62.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

The washed slurry obtained by the procedure 3 was dropped onto acollodion film on a grid and adhered thereto, and then, naturally dried.Thereafter, carbon vapor deposition was performed, and the resultant wassubjected to TEM observation. In FIG. 1, a TEM photograph of theprecursor particles (the particles of the substituent element-containingiron oxyhydroxide) obtained in this Example is shown. Note that thelength indicated by the white horizontal line displayed in the lowerright of the TEM photograph is 100 nm. In addition, when the DLS averagediameter of the precursor particles contained in the washed slurryobtained by the procedure 3 was measured, the DLS average diameterthereof was 48.4 nm.

Subsequently, 704.7 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 807 g. The temperature was adjusted to 40° C. while stirring, and945.2 g of 2-propanol was added thereto, followed by stirring for 5minutes, and thereafter, 207.0 g of a 22.3 mass % ammonia aqueoussolution was added thereto. Further, 829.2 g of tetraethoxysilane (TEAS)as a silicon compound having a hydrolyzable group was added thereto over35 minutes. Thereafter, while keeping the liquid temperature at 40° C.,stirring was continued as such for 1 hour to carry out coating with ahydrolysate of the silicon compound. Thereafter, the obtained solutionwas washed and subjected to solid-liquid separation, and the product wascollected as a cake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1050° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 24 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 25 mass % TMAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 5.9% was obtained. Here, the additionamount of the TMAOH aqueous solution was set so that the TMAOHconcentration in the surface modifier-containing slurry became 0.065mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of16,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 31,000 G. For a slurry on theprecipitate side, the operation of addition of 30 g of a 0.065 mol/kgTMAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 16,000 rpm and 31,000 G for 30 minutes, and collection of30 g of a supernatant slurry was repeated further 9 times, whereby atotal of 300 g of the supernatant slurry was collected. After 300 g ofthe collected supernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry containingfine particles was removed and a slurry on the precipitate side wasobtained. The gravitational acceleration in the centrifugation treatmentwas set to 39,000 G. To a slurry on the precipitate side obtained byrepeating the operation of addition of 30 g of a 0.065 mol/kg TMAOHaqueous solution, ultrasonic dispersion, a centrifugation treatment at18,000 rpm and 39,000 G for 30 minutes, and removal of 30 g of asupernatant slurry further 9 times for the obtained slurry on theprecipitate side, a 1 mass % sulfuric acid aqueous solution was added toadjust the pH to 6.5 and aggregate substitution-type ε-iron oxideparticles, followed by filtration through a membrane filter, and thesolid was collected and then dried, whereby an iron-based oxide magneticpowder according to Example 2 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of317 (kA/m), a saturation magnetization of 14.7 (Am²/kg), a square ratioof 0.590, an SFD of 1.43, and an I_(L)/I_(H) value of 0.56. Further, asa result of subjecting the obtained iron-based oxide magnetic powder toTEM observation by the same procedure as in Example 1, the TEM averageparticle diameter was 14.2 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.5%, the number ratio ofparticles with a particle diameter of 20 nm or more was 7.2%, thecoefficient of variation (CV value) was 19%, and the average circularityof the particles was 0.960. In FIG. 2, a TEM photograph of theiron-based oxide magnetic powder obtained in this Example is shown. Notethat the length indicated by the white horizontal line displayed in thelower right of the TEM photograph is 100 nm. In addition, evaluation ofcompositional analysis was performed, and the molar compositional ratioof each metal element was calculated when the total amount of iron andthe substituent metal elements was set to 2 mol.

Example 3

As an iron raw material containing chloride ions, 414.1 g of ferric(III)chloride hexahydrate with a purity of 99 mass %, as raw materials ofsubstituent elements, 39.7 g of a gallium(III) nitrate solution with aGa concentration of 10.9 mass % and a nitrate concentration of 31 mass%, 11.2 g of cobalt(II) nitrate hexahydrate with a purity of 97 mass %,and 10.8 g of a titanium(IV) chloride solution with a Ti concentrationof 16.5 mass % and a chloride ion concentration of 31 mass %, and 641.5g of pure water were mixed, whereby a raw material aqueous solution wasprepared. The molar ratio of metal ions in the raw material aqueoussolution is as follows: Fe:Ga:Co:Ti=1.835:0.075:0.045:0.045.Subsequently, in a 5 L reaction vessel, 1500 g of pure water was placed,and the liquid temperature was adjusted to 40° C. while mechanicallystirring with a stirring blade. Subsequently, while continuing stirringand keeping the liquid temperature at 40° C., the raw material aqueoussolution and a 6.2% ammonia aqueous solution (as an alkali) weresimultaneously added into the pure water, and thereafter, stirring wasperformed for 60 minutes while keeping the liquid temperature at 40° C.,whereby a colloidal solution was obtained (procedure 1, firstneutralization step). Here, the raw material aqueous solution wascontinuously added over 240 minutes at an addition rate of 4.21 g/min,and the ammonia aqueous solution was continuously added over 240 minutesat an addition rate of 4.21 g/min. While adding the raw material aqueoussolution and the ammonia aqueous solution, the ratio of the cumulativeaddition amount of the alkali to the total cumulative addition amount ofthe acid group contained in the raw material solution was 0.75equivalents. The pH of the obtained colloidal solution was 2.11.

Subsequently, to the colloidal solution obtained by the procedure 1,288.8 g of a citric acid solution with a citric acid concentration of 20mass % as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 186.1 g of a 22.14% ammonia solution was addedthereto all at once, and then, the resulting mixture was held for 10minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding186.1 g of the ammonia solution all at once was 8.88.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 35.5 nm.

Subsequently, 848.1 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 1000 g. The temperature was adjusted to 40° C. while stirring, and1171.3 g of ethanol was added thereto, followed by stirring for 5minutes, and thereafter, 257.8 g of a 22.14 mass % ammonia aqueoussolution was added thereto. Further, 1027.5 g of tetraethoxysilane(TEAS) as a silicon compound having a hydrolyzable group was addedthereto over 35 minutes. Thereafter, while keeping the liquidtemperature at 40° C., stirring was continued as such for 1 hour tocarry out coating with a hydrolysate of the silicon compound.Thereafter, the obtained solution was washed and subjected tosolid-liquid separation, and the product was collected as a cake(procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1110° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 24 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 25 mass % TMAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 5.7% was obtained. Here, the additionamount of the TMAOH aqueous solution was set so that the TMAOHconcentration in the surface modifier-containing slurry became 0.065mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of16,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 31,000 G. For a slurry on theprecipitate side, the operation of addition of 30 g of a 0.065 mol/kgTMAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 16,000 rpm and 31,000 G for 30 minutes, and collection of30 g of a supernatant slurry was repeated further 9 times, whereby atotal of 300 g of the supernatant slurry was collected. After 300 g ofthe collected supernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry containingfine particles was removed and a slurry on the precipitate side wasobtained. The gravitational acceleration in the centrifugation treatmentwas set to 39,000 G. To a slurry on the precipitate side obtained byrepeating the operation of addition of 30 g of a 0.065 mol/kg TMAOHaqueous solution, ultrasonic dispersion, a centrifugation treatment at18,000 rpm and 39,000 G for 30 minutes, and removal of 30 g of asupernatant slurry further 9 times for the obtained slurry on theprecipitate side, a 1 mass % sulfuric acid aqueous solution was added toadjust the pH to 6.5 and aggregate substitution-type ε-iron oxideparticles, followed by filtration through a membrane filter, and thesolid was collected and then dried, whereby an iron-based oxide magneticpowder according to Example 3 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of441 (kA/m), a saturation magnetization of 12.8 (Am²/kg), a square ratioof 0.645, an SFD of 1.08, and an I_(L)/I_(H) value of 0.42. Further, asa result of subjecting the obtained iron-based oxide magnetic powder toTEM observation by the same procedure as in Example 1, the TEM averageparticle diameter was 14.1 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.0%, the number ratio ofparticles with a particle diameter of 20 nm or more was 0.0%, thecoefficient of variation (CV value) was 14%, and the average circularityof the particles was 0.972. In FIG. 3, a TEM photograph of theiron-based oxide magnetic powder obtained in this Example is shown. Notethat the length indicated by the white horizontal line displayed in thelower right of the TEM photograph is 100 nm. In addition, evaluation ofcompositional analysis was performed, and the molar compositional ratioof each metal element was calculated when the total amount of iron andthe substituent metal elements was set to 2 mol.

Example 4

As an iron raw material containing chloride ions, 391.9 g of ferric(III)chloride hexahydrate with a purity of 99 mass %, as raw materials ofsubstituent elements, 10.1 g of cobalt(II) nitrate hexahydrate with apurity of 97 mass %, and 9.8 g of a titanium(IV) chloride solution witha Ti concentration of 16.5 mass % and a chloride ion concentration of 31mass %, and 667.4 g of pure water were mixed, whereby a raw materialaqueous solution was prepared. The molar ratio of metal ions in the rawmaterial aqueous solution is as follows: Fe:Co:Ti=1.910:0.045:0.045.Subsequently, in a 5 L reaction vessel, 1500 g of pure water was placed,and the liquid temperature was adjusted to 40° C. while mechanicallystirring with a stirring blade. Subsequently, while continuing stirringand keeping the liquid temperature at 40° C., the raw material aqueoussolution and a 5.6% ammonia aqueous solution (as an alkali) weresimultaneously added into the pure water, and thereafter, stirring wasperformed for 60 minutes while keeping the liquid temperature at 40° C.,whereby a colloidal solution was obtained (procedure 1, firstneutralization step). Here, the raw material aqueous solution wascontinuously added over 240 minutes at an addition rate of 4.22 g/min,and the ammonia aqueous solution was continuously added over 240 minutesat an addition rate of 4.22 g/min. While adding the raw material aqueoussolution and the ammonia aqueous solution, the ratio of the cumulativeaddition amount of the alkali to the total cumulative addition amount ofthe acid group contained in the raw material solution was 0.75equivalents. The pH of the obtained colloidal solution was 2.00.

Subsequently, to the colloidal solution obtained by the procedure 1,288.8 g of a citric acid solution with a citric acid concentration of 20mass % as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 185.4 g of a 22.14% ammonia solution was addedthereto all at once, and then, the resulting mixture was held for 10minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding185.4 g of the ammonia solution all at once was 8.90.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 36.9 nm.

Subsequently, 848.8 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 1000 g. The temperature was adjusted to 40° C. while stirring, and1171.3 g of ethanol was added thereto, followed by stirring for 5minutes, and thereafter, 257.8 g of a 22.14 mass % ammonia aqueoussolution was added thereto. Further, 1027.5 g of tetraethoxysilane(TEOS) as a silicon compound having a hydrolyzable group was addedthereto over 35 minutes. Thereafter, while keeping the liquidtemperature at 40° C., stirring was continued as such for 1 hour tocarry out coating with a hydrolysate of the silicon compound.Thereafter, the obtained solution was washed and subjected tosolid-liquid separation, and the product was collected as a cake(procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1110° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 24 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 25 mass % TMAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 5.7% was obtained. Here, the additionamount of the TMAOH aqueous solution was set so that the TMAOHconcentration in the surface modifier-containing slurry became 0.065mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of16,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 31,000 G. For a slurry on theprecipitate side, the operation of addition of 30 g of a 0.065 mol/kgTMAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 16,000 rpm and 31,000 G for 30 minutes, and collection of30 g of a supernatant slurry was repeated further 9 times, whereby atotal of 300 g of the supernatant slurry was collected. After 300 g ofthe collected supernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry containingfine particles was removed and a slurry on the precipitate side wasobtained. The gravitational acceleration in the centrifugation treatmentwas set to 39,000 G. To a slurry on the precipitate side obtained byrepeating the operation of addition of 30 g of a 0.065 mol/kg TMAOHaqueous solution, ultrasonic dispersion, a centrifugation treatment at18,000 rpm and 39,000 G for 30 minutes, and removal of 30 g of asupernatant slurry further 9 times for the obtained slurry on theprecipitate side, a 1 mass % sulfuric acid aqueous solution was added toadjust the pH to 6.5 and aggregate substitution-type ε-iron oxideparticles, followed by filtration through a membrane filter, and thesolid was collected and then dried, whereby an iron-based oxide magneticpowder according to Example 4 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of558 (kA/m), a saturation magnetization of 11.4 (Am²/kg), a square ratioof 0.673, an SFD of 0.89, and an I_(L)/I_(H) value of 0.29. Further, asa result of subjecting the obtained iron-based oxide magnetic powder toTEM observation by the same procedure as in Example 1, the TEM averageparticle diameter was 14.2 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.0%, the number ratio ofparticles with a particle diameter of 20 nm or more was 0.0%, thecoefficient of variation (CV value) was 12%, and the average circularityof the particles was 0.972. In FIG. 4, a TEM photograph of theiron-based oxide magnetic powder obtained in this Example is shown. Notethat the length indicated by the white horizontal line displayed in thelower right of the TEM photograph is 100 nm. In addition, evaluation ofcompositional analysis was performed, and the molar compositional ratioof each metal element was calculated when the total amount of iron andthe substituent metal elements was set to 2 mol.

Example 5

As an iron raw material containing chloride ions, 354.9 g of ferric(III)chloride hexahydrate with a purity of 99 mass, as raw materials ofsubstituent elements, 72.1 g of a gallium(III) nitrate solution with aGa concentration of 10.9 mass % and a nitrate concentration of 33 mass%, 15.8 g of cobalt(II) nitrate hexahydrate with a purity of 97 mass,and 10.9 g of a titanium(IV) chloride solution with a Ti concentrationof 16.5 mass % and a chloride ion concentration of 31 mass %, and 475.6g of pure water were mixed, whereby a raw material aqueous solution wasprepared. The molar ratio of metal ions in the raw material aqueoussolution is as follows: Fe:Ga:Co:Ti=1.730:0.150:0.070:0.050.Subsequently, in a 5 L reaction vessel, 1500 g of pure water was placed,and the liquid temperature was adjusted to 40° C. while mechanicallystirring with a stirring blade. Subsequently, while continuing stirringand keeping the liquid temperature at 40° C., the raw material aqueoussolution and a 6.2% ammonia aqueous solution (as an alkali) weresimultaneously added into the pure water, and thereafter, stirring wasperformed for 60 minutes while keeping the liquid temperature at 40° C.,whereby a colloidal solution was obtained (procedure 1, firstneutralization step). Here, the raw material aqueous solution wascontinuously added over 240 minutes at an addition rate of 3.87 g/min,and the ammonia aqueous solution was continuously added over 240 minutesat an addition rate of 3.87 g/min. While adding the raw material aqueoussolution and the ammonia aqueous solution, the ratio of the cumulativeaddition amount of the alkali to the total cumulative addition amount ofthe acid group contained in the raw material solution was 0.75equivalents. The pH of the obtained colloidal solution was 1.65.

Subsequently, to the colloidal solution obtained by the procedure 1,288.8 g of a citric acid solution with a citric acid concentration of 20mass % as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 186.3 g of a 22.14% ammonia solution was addedthereto all at once, and then, the resulting mixture was held for 10minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding186.3 g of the ammonia solution all at once was 8.43.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 37.4 nm.

Subsequently, 997.6 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 1100 g. The temperature was adjusted to 40° C. while stirring, and1288.4 g of ethanol was added thereto, followed by stirring for 5minutes, and thereafter, 283.6 g of a 22.14 mass % ammonia aqueoussolution was added thereto. Further, 1130.3 g of tetraethoxysilane(TEOS) as a silicon compound having a hydrolyzable group was addedthereto over 35 minutes. Thereafter, while keeping the liquidtemperature at 40° C., stirring was continued as such for 1 hour tocarry out coating with a hydrolysate of the silicon compound.Thereafter, the obtained solution was washed and subjected tosolid-liquid separation, and the product was collected as a cake(procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1130° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 24 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 25 mass % TMAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 4.1% was obtained. Here, the additionamount of the TMAOH aqueous solution was set so that the TMAOHconcentration in the surface modifier-containing slurry became 0.065mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of14,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 24,000 G. For a slurry on theprecipitate side, the operation of addition of 30 g of a 0.065 mol/kgTMAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 14,000 rpm and 24,000 G for 30 minutes, and collection of30 g of a supernatant slurry was repeated further 9 times, whereby atotal of 300 g of the supernatant slurry was collected. After 300 g ofthe collected supernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry containingfine particles was removed and a precipitate was obtained. Thegravitational acceleration in the centrifugation treatment was set to39,000 G. To a slurry on the precipitate side obtained by repeating theoperation of addition of 30 g of a 0.065 mol/kg TMAOH aqueous solution,ultrasonic dispersion, a centrifugation treatment at 18,000 rpm and39,000 G for 30 minutes, and removal of 30 g of a supernatant slurryfurther 9 times for the obtained slurry on the precipitate side, 1 mass% sulfuric acid aqueous solution was added to adjust the pH to 6.5 andaggregate substitution-type ε-iron oxide particles, followed byfiltration through a membrane filter, and the solid was collected andthen dried, whereby an iron-based oxide magnetic powder according toExample 5 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of341 (kA/m), a saturation magnetization of 16.2 (Am²/kg), a square ratioof 0.641, an SFD of 0.69, and an I_(L)/I_(H) value of 0.14. Further, asa result of subjecting the obtained iron-based oxide magnetic powder toTEM observation by the same procedure as in Example 1, the TEM averageparticle diameter was 15.3 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.1%, the number ratio ofparticles with a particle diameter of 20 nm or more was 0.0%, thecoefficient of variation (CV value) was 13%, and the average circularityof the particles was 0.973. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 6

As an iron raw material containing chloride ions, 329.3 g of ferric(III)chloride hexahydrate with a purity of 99 mass %, as raw materials ofsubstituent elements, 132.2 g of a gallium(III) nitrate solution with aGa concentration of 10.9 mass % and a nitrate concentration of 33 mass%, 15.8 g of cobalt(II) nitrate hexahydrate with a purity of 97 mass,and 10.9 g of a titanium(IV) chloride solution with a Ti concentrationof 16.5 mass % and a chloride ion concentration of 31 mass %, and 441.5g of pure water were mixed, whereby a raw material aqueous solution wasprepared. The molar ratio of metal ions in the raw material aqueoussolution is as follows: Fe:Ga:Co:Ti=1.650:0.275:0.070:0.050.Subsequently, in a 5 L reaction vessel, 1500 g of pure water was placed,and the liquid temperature was adjusted to 40° C. while mechanicallystirring with a stirring blade. Subsequently, while continuing stirringand keeping the liquid temperature at 40° C., the raw material aqueoussolution and a 6.2% ammonia aqueous solution (as an alkali) weresimultaneously added into the pure water, and thereafter, stirring wasperformed for 60 minutes while keeping the liquid temperature at 40° C.,whereby a colloidal solution was obtained (procedure 1, firstneutralization step). Here, the raw material aqueous solution wascontinuously added over 240 minutes at an addition rate of 3.87 g/min,and the ammonia aqueous solution was continuously added over 240 minutesat an addition rate of 3.87 g/min. While adding the raw material aqueoussolution and the ammonia aqueous solution, the ratio of the cumulativeaddition amount of the alkali to the total cumulative addition amount ofthe acid group contained in the raw material solution was 0.75equivalents. The pH of the obtained colloidal solution was 1.66.

Subsequently, to the colloidal solution obtained by the procedure 1,288.8 g of a citric acid solution with a citric acid concentration of 20mass % as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 186.8 g of a 22.25% ammonia solution was addedthereto all at once, and then, the resulting mixture was held for 10minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding186.8 g of the ammonia solution all at once was 8.51.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 32.5 nm.

Subsequently, 971.0 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 1100 g. The temperature was adjusted to 40° C. while stirring, and1288.4 g of ethanol was added thereto, followed by stirring for 5minutes, and thereafter, 283.6 g of a 22.14 mass % ammonia aqueoussolution was added thereto. Further, 1130.3 g of tetraethoxysilane(TEOS) as a silicon compound having a hydrolyzable group was addedthereto over 35 minutes. Thereafter, while keeping the liquidtemperature at 40° C., stirring was continued as such for 1 hour tocarry out coating with a hydrolysate of the silicon compound.Thereafter, the obtained solution was washed and subjected tosolid-liquid separation, and the product was collected as a cake(procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1130° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 24 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 25 mass % TMAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 4.3% was obtained. Here, the additionamount of the TMAOH aqueous solution was set so that the TMAOHconcentration in the surface modifier-containing slurry became 0.065mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of14,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 24,000 G. For a slurry on theprecipitate side, the operation of addition of 30 g of a 0.065 mol/kgTMAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 14,000 rpm and 24,000 G for 30 minutes, and collection of30 g of a supernatant slurry was repeated further 9 times, whereby atotal of 300 g of the supernatant slurry was collected. After 300 g ofthe collected supernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry containingfine particles was removed and a slurry on the precipitate side wasobtained. The gravitational acceleration in the centrifugation treatmentwas set to 39,000 G. To a slurry on the precipitate side obtained byrepeating the operation of addition of 30 g of a 0.065 mol/kg TMAOHaqueous solution, ultrasonic dispersion, a centrifugation treatment at18,000 rpm and 39,000 G for 30 minutes, and removal of 30 g of asupernatant slurry further 9 times for the obtained slurry on theprecipitate side, a 1 mass % sulfuric acid aqueous solution was added toadjust the pH to 6.5 and aggregate substitution-type ε-iron oxideparticles, followed by filtration through a membrane filter, and thesolid was collected and then dried, whereby an iron-based oxide magneticpowder according to Example 6 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of200 kA/m), a saturation magnetization of 17.4 (Am²/kg), a square ratioof 0.577, an SFD of 1.02, and an I_(L)/I_(H) value of 0.27. Further, asa result of subjecting the obtained iron-based oxide magnetic powder toTEM observation by the same procedure as in Example 1, the TEM averageparticle diameter was 15.7 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.0%, the number ratio ofparticles with a particle diameter of 20 nm or more was 1.0%, thecoefficient of variation (CV value) was 15%, and the average circularityof the particles was 0.970. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 7

As an iron raw material containing chloride ions, 334.4 g of ferric(III)chloride hexahydrate with a purity of 99 mass %, as raw materials ofsubstituent elements, 120.2 g of a gallium(III) nitrate solution with aGa concentration of 10.9 mass % and a nitrate concentration of 33 mass%, 15.8 g of cobalt(II) nitrate hexahydrate with a purity of 97 mass %,and 10.9 g of a titanium(IV) chloride solution with a Ti concentrationof 16.5 mass % and a chloride ion concentration of 31 mass %, and 448.2g of pure water were mixed, whereby a raw material aqueous solution wasprepared. The molar ratio of metal ions in the raw material aqueoussolution is as follows: Fe:Ga:Co:Ti=1.630:0.250:0.070:0.050.Subsequently, in a 5 L reaction vessel, 1500 g of pure water was placed,and the liquid temperature was adjusted to 40° C. while mechanicallystirring with a stirring blade. Subsequently, while continuing stirringand keeping the liquid temperature at 40° C., the raw material aqueoussolution and a 6.2% ammonia aqueous solution (as an alkali) weresimultaneously added into the pure water, and thereafter, stirring wasperformed for 60 minutes while keeping the liquid temperature at 40° C.,whereby a colloidal solution was obtained (procedure 1, firstneutralization step). Here, the raw material aqueous solution wascontinuously added over 240 minutes at an addition rate of 3.87 g/min,and the ammonia aqueous solution was continuously added over 240 minutesat an addition rate of 3.87 g/min. While adding the raw material aqueoussolution and the ammonia aqueous solution, the ratio of the cumulativeaddition amount of the alkali to the total cumulative addition amount ofthe acid group contained in the raw material solution was 0.75equivalents. The pH of the obtained colloidal solution was 1.61.

Subsequently, to the colloidal solution obtained by the procedure 1,288.8 g of a citric acid solution with a citric acid concentration of 20mass % as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 186.5 g of a 22.25% ammonia solution was addedthereto all at once, and then, the resulting mixture was held for 10minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding186.5 g of the ammonia solution all at once was 8.43.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 32.3 nm.

Subsequently, 923.2 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 1100 g. The temperature was adjusted to 40° C. while stirring, and1288.4 g of ethanol was added thereto, followed by stirring for 5minutes, and thereafter, 283.6 g of a 22.14 mass % ammonia aqueoussolution was added thereto. Further, 1130.3 g of tetraethoxysilane(TEOS) as a silicon compound having a hydrolyzable group was addedthereto over 35 minutes. Thereafter, while keeping the liquidtemperature at 40° C., stirring was continued as such for 1 hour tocarry out coating with a hydrolysate of the silicon compound.Thereafter, the obtained solution was washed and subjected tosolid-liquid separation, and the product was collected as a cake(procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1130° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 24 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 25 mass % TMAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 4.3% was obtained. Here, the additionamount of the TMAOH aqueous solution was set so that the TMAOHconcentration in the surface modifier-containing slurry became 0.065mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of14,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 24,000 G. For a slurry on theprecipitate side, the operation of addition of 30 g of a 0.065 mol/kgTMAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 14,000 rpm and 24,000 G for 30 minutes, and collection of30 g of a supernatant slurry was repeated further 9 times, whereby atotal of 300 g of the supernatant slurry was collected. After 300 g ofthe collected supernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry containingfine particles was removed and a slurry on the precipitate side wasobtained. The gravitational acceleration in the centrifugation treatmentwas set to 39,000 G. To a slurry on the precipitate side obtained byrepeating the operation of addition of 30 g of a 0.065 mol/kg TMAOHaqueous solution, ultrasonic dispersion, a centrifugation treatment at18,000 rpm and 39,000 G for 30 minutes, and removal of 30 g of asupernatant slurry further 9 times for the obtained slurry on theprecipitate side, a 1 mass % sulfuric acid aqueous solution was added toadjust the pH to 6.5 and aggregate substitution-type ε-iron oxideparticles, followed by filtration through a membrane filter, and thesolid was collected and then dried, whereby an iron-based oxide magneticpowder according to Example 7 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of223 (kA/m), a saturation magnetization of 17.4 (Am²/kg), a square ratioof 0.595, an SFD of 0.91, and an value of 0.21. Further, as a result ofsubjecting the obtained iron-based oxide magnetic powder to TEMobservation by the same procedure as in Example 1, the TEM averageparticle diameter was 15.5 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.0%, the number ratio ofparticles with a particle diameter of 20 nm or more was 0.5%, thecoefficient of variation (CV value) was 14%, and the average circularityof the particles was 0.972. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 8

As an iron raw material containing chloride ions, 627.8 g of ferric(III)chloride hexahydrate with a purity of 99 mass %, as raw materials ofsubstituent elements, 333.4 g of a gallium(III) nitrate solution with aGa concentration of 11.0 mass % and a nitrate concentration of 33 mass%, 31.6 g of cobalt(II) nitrate hexahydrate with a purity of 97 mass %,and 21.8 g of a titanium(IV) chloride solution with a Ti concentrationof 16.5 mass % and a chloride ion concentration of 31 mass %, and 814.3g of pure water were mixed, whereby a raw material aqueous solution wasprepared. The molar ratio of metal ions in the raw material aqueoussolution is as follows: Fe:Ga:Co:Ti=1.530:0.350:0.070:0.050.Subsequently, in a 10 L reaction vessel, 3000 g of pure water wasplaced, and the liquid temperature was adjusted to 40° C. whilemechanically stirring with a stirring blade. Subsequently, whilecontinuing stirring and keeping the liquid temperature at 40° C., theraw material aqueous solution and a 6.8% ammonia aqueous solution (as analkali) were simultaneously added into the pure water, and thereafter,stirring was performed for 60 minutes while keeping the liquidtemperature at 40° C., whereby a colloidal solution was obtained(procedure 1, first neutralization step). Here, the raw material aqueoussolution was continuously added over 240 minutes at an addition rate of7.62 g/min, and the ammonia aqueous solution was continuously added over240 minutes at an addition rate of 7.62 g/min. While adding the rawmaterial aqueous solution and the ammonia aqueous solution, the ratio ofthe cumulative addition amount of the alkali to the total cumulativeaddition amount of the acid group contained in the raw material solutionwas 0.80 equivalents. The pH of the obtained colloidal solution was1.86.

Subsequently, to the colloidal solution obtained by the procedure 1,577.5 g of a citric acid solution with a citric acid concentration of 20mass % as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 376.6 g of a 22.13% ammonia solution was addedthereto all at once, and then, the resulting mixture was held for 10minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding376.6 g of the ammonia solution all at once was 8.6.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 50.8 nm.

Subsequently, 1975.5 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 2500 g. The temperature was adjusted to 40° C. while stirring, and2928.2 g of ethanol was added thereto, followed by stirring for 5minutes, and thereafter, 702.0 g of a 21.87 mass % ammonia aqueoussolution was added thereto. Further, 2763.9 g of tetraethoxysilane(TEOS) as a silicon compound having a hydrolyzable group was addedthereto over 35 minutes. Thereafter, while keeping the liquidtemperature at 40° C., stirring was continued as such for 1 hour tocarry out coating with a hydrolysate of the silicon compound.Thereafter, the obtained solution was washed and subjected tosolid-liquid separation, and the product was collected as a cake(procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1116° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 24 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 25 mass % TMAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 35% was obtained. Here, the additionamount of the TMAOH aqueous solution was set so that the TMAOHconcentration in the surface modifier-containing slurry became 0.065mol/kg. In this case, the pH of the slurry became 13.

550 g of the obtained surface modifier-containing slurry was subjectedto an ultrasonic dispersion treatment for 1 hour with an ultrasoniccleaner (Yamato 5510 Branson, manufactured by (Yamato Scientific) Co.,Ltd.), and then subjected to a centrifugation treatment at a rotationspeed of 8800 rpm for 130 minutes with an R10A3 rotor of a centrifuge(himac CR21GII, manufactured by Hitachi Koki Co., Ltd.), and 250 g of asupernatant slurry was removed. The gravitational acceleration in thecentrifugation treatment was set to 15,000 G. For a slurry on theprecipitate side, the operation of addition of 250 g of a 0.065 mol/kgTMAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 8800 rpm and 15,000 G for 130 minutes, and removal of 250 gof a supernatant slurry was repeated further times, whereby a slurry onthe precipitate side was obtained (procedure 7).

Subsequently, 250 of the 0.065 mol/kg TMAOH aqueous solution was addedto 300 g of the slurry on the precipitate side obtained by the procedure7, and thereafter, the resulting mixture was subjected to an ultrasonicdispersion treatment for 1 hour with an ultrasonic cleaner (Yamato 5510Branson, manufactured by (Yamato Scientific) Co., Ltd.), and thensubjected to a centrifugation treatment at a rotation speed of 8800 rpmfor 59 minutes with an R10A3 rotor of a centrifuge (himac CR21GII,manufactured by Hitachi Koki Co., Ltd.), and 250 g of a supernatantslurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 15,000 G. For a slurry on theprecipitate side, the operation of addition of 250 g of a 0.065 mol/kgTMAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 8800 rpm and 15,000 G for 59 minutes, and collection of 250g of a supernatant slurry was repeated further 12 times, whereby a totalof 3250 g of the supernatant slurry was collected. After 3250 g of thecollected supernatant slurry was mixed, a 1 mass % sulfuric acid aqueoussolution was added to adjust the pH to 6.5 and aggregatesubstitution-type ε-iron oxide particles, followed by filtration througha membrane filter, and the solid was collected and then dried, wherebyan iron-based oxide magnetic powder according to Example 8 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of183 (kA/m), a saturation magnetization of 17.7 (Am²/kg), a square ratioof 0.558, an SFS of 1.19, and an I_(L)/I_(H) value of 0.32. Further, asa result of subjecting the obtained iron-based oxide magnetic powder toTEM observation by the same procedure as in Example 1, the TEM averageparticle diameter was 15.0 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.3%, the number ratio ofparticles with a particle diameter of 20 nm or more was 1.0%, thecoefficient of variation (CV value) was 14%, and the average circularityof the particles was 0.974. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 9

As an iron raw material containing chloride ions, 342.6 g of ferric(III)chloride hexahydrate with a purity of 99 mass %, as raw materials ofsubstituent elements, 114.3 g of a gallium(III) nitrate solution with aGa concentration of 11.0 mass % and a nitrate concentration of 33 mass%, 10.1 g of cobalt(II) nitrate hexahydrate with a purity of 97 mass %,and 9.8 g of a titanium(IV) chloride solution with a Ti concentration of16.5 mass % and a chloride ion concentration of 31 mass %, and 464.0 gof pure water were mixed, whereby a raw material aqueous solution wasprepared. The molar ratio of metal ions in the raw material aqueoussolution is as follows: Fe:Ga:Co:Ti=1.670:0.240:0.045:0.045.Subsequently, in a 5 L reaction vessel, 1500 g of pure water was placed,and the liquid temperature was adjusted to 40° C. while mechanicallystirring with a stirring blade. Subsequently, while continuing stirringand keeping the liquid temperature at 40° C., the raw material aqueoussolution and a 6.5% ammonia aqueous solution (as an alkali) weresimultaneously added into the pure water, and thereafter, stirring wasperformed for 60 minutes while keeping the liquid temperature at 40° C.,whereby a colloidal solution was obtained (procedure 1, firstneutralization step). Here, the raw material aqueous solution wascontinuously added over 240 minutes at an addition rate of 3.92 g/min,and the ammonia aqueous solution was continuously added over 240 minutesat an addition rate of 3.92 g/min. While adding the raw material aqueoussolution and the ammonia aqueous solution, the ratio of the cumulativeaddition amount of the alkali to the total cumulative addition amount ofthe acid group contained in the raw material solution was 0.79equivalents. The pH of the obtained colloidal solution was 1.81.

Subsequently, to the colloidal solution obtained by the procedure 1,288.8 g of a citric acid solution with a citric acid concentration of 20mass % as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 190.2 g of a 21.87% ammonia solution was addedthereto all at once, and then, the resulting mixture was held for 10minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding190.2 g of the ammonia solution all at once was 8.67.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 58.1 nm.

Subsequently, 695.5 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 807 g. The temperature was adjusted to 40° C. while stirring, and945.2 g of ethanol was added thereto, followed by stirring for 5minutes, and thereafter, 207.02 g of a 22.25 mass % ammonia aqueoussolution was added thereto. Further, 829.2 g of tetraethoxysilane (TEOS)as a silicon compound having a hydrolyzable group was added thereto over35 minutes. Thereafter, while keeping the liquid temperature at 40° C.,stirring was continued as such for 1 hour to carry out coating with ahydrolysate of the silicon compound. Thereafter, the obtained solutionwas washed and subjected to solid-liquid separation, and the product wascollected as a cake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1050° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 24 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 25 mass % TMAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 4.1% was obtained. Here, the additionamount of the TMAOH aqueous solution was set so that the TMAOHconcentration in the surface modifier-containing slurry became 0.065mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of20,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was removed. The gravitational acceleration in thecentrifugation treatment was set to 48,000 G. For a slurry on theprecipitate side, the operation of addition of 30 g of a 0.065 mol/kgTMAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 20,000 rpm and 48,000 G for 30 minutes, and removal of 30 gof a supernatant slurry was repeated further times, whereby a slurry onthe precipitate side was obtained (procedure 7).

After 30 g of a 0.065 mol/kg TMAOH aqueous solution was added to 10 g ofthe slurry on the precipitate side obtained by the procedure 7, theresulting mixture was subjected to an ultrasonic dispersion treatmentfor 1 hour with an ultrasonic cleaner (Yamato 5510 Branson, manufacturedby (Yamato Scientific) Co., Ltd.), and then subjected to acentrifugation treatment at a rotation speed of 19,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Kaki Co., Ltd.), and 30 g of a supernatant slurry wasremoved. The gravitational acceleration in the centrifugation treatmentwas set to 43,000 G. For a slurry on the precipitate side, the operationof addition of 250 g of a 0.065 mol/kg TMAOH aqueous solution,ultrasonic dispersion, a centrifugation treatment at 19,000 rpm and43,000 G for 30 minutes, and removal of 30 g of a supernatant slurry wasrepeated further times, whereby a slurry on the precipitate side wasobtained (procedure 8).

After 30 g of a 0.065 mol/kg TMAOH aqueous solution was added to 10 g ofthe slurry on the precipitate side obtained by the procedure 8, theresulting mixture was subjected to an ultrasonic dispersion treatmentfor 1 hour with an ultrasonic cleaner (Yamato 5510 Branson, manufacturedby (Yamato Scientific) Co., Ltd.), and then subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry wascollected. The gravitational acceleration in the centrifugationtreatment was set to 39,000 G. For a slurry on the precipitate side, theoperation of addition of 30 g of a 0.065 mol/kg TMAOH aqueous solution,ultrasonic dispersion, a centrifugation treatment at 18,000 rpm and39,000 G for 30 minutes, and collection of 30 g of a supernatant slurrywas repeated further 9 times, whereby a total of 300 g of thesupernatant slurry was obtained. After 300 g of the collectedsupernatant slurry was mixed, a 1 mass % sulfuric acid aqueous solutionwas added to adjust the pH to 6.5 and aggregate substitution-type ε-ironoxide particles, followed by filtration through a membrane filter, andthe solid was collected and then dried, whereby an iron-based oxidemagnetic powder according to Example 9 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of272 (kA/m), a saturation magnetization of 15.0 (Am²/kg), a square ratioof 0.606, an SFD of 1.16, and an I_(L)/I_(H) value of 0.34. Further, asa result of subjecting the obtained iron-based oxide magnetic powder toTEM observation by the same procedure as in Example 1, the TEM averageparticle diameter was 14.0 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.3%, the number ratio ofparticles with a particle diameter of 20 nm or more was 0.6%, thecoefficient of variation (CV value) was 14%, and the average circularityof the particles was 0.966. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Comparative Example 1

As an iron raw material containing chloride ions, 313.9 g of ferric(III)chloride hexahydrate with a purity of 99 mass %, as raw materials ofsubstituent elements, 195.1 g of a gallium(III) nitrate solution with aGa concentration of 9.4 mass % and a nitrate concentration of 24 mass %,15.8 g of cobalt(II) nitrate hexahydrate with a purity of 97 mass, and10.9 g of a titanium(IV) chloride solution with a Ti concentration of16.5 mass % and a chloride ion concentration of 31 mass %, and 398.0 gof pure water were mixed, whereby a raw material solution was prepared.The molar ratio of metal ions in the raw material solution is asfollows: Fe:Ga:Co:Ti=1.530:0.350:0.070:0.050. Subsequently, in a 5 Lreaction vessel, 1500 g of pure water was placed, and the liquidtemperature was adjusted to 40° C. while mechanically stirring with astirring blade. Subsequently, while continuing stirring and keeping theliquid temperature at 40° C., the raw material aqueous solution and a6.4 mass % ammonia aqueous solution as an alkaline aqueous solution weresimultaneously added into the pure water, and thereafter, stirring wasperformed for 60 minutes while keeping the liquid temperature at 40° C.,whereby a colloidal solution was obtained (procedure 1, firstneutralization step). Here, the raw material aqueous solution wascontinuously added over 240 minutes at an addition rate of 3.9 g/min,and the ammonia aqueous solution was continuously added over 240 minutesat an addition rate of 3.9 g/min. While adding the raw material solutionand the ammonia aqueous solution, the ratio of the cumulative additionamount of the alkali to the total cumulative addition amount of the acidgroup contained in the raw material solution was 0.80 equivalents. ThepH of the obtained colloidal solution was 1.8.

Subsequently, to the colloidal solution obtained by the procedure 1,288.75 g of a citric acid solution with a citric acid concentration of20 mass % as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 152.86 g of a 22.4 mass % ammonia aqueoussolution was added thereto all at once, and then, the resulting mixturewas held for 10 minutes while stirring under the condition oftemperature of 40° C., whereby a slurry of precursor particles, which isan intermediate product, was obtained (procedure 2, secondneutralization step). The pH after adding 152.86 g of the ammoniasolution all at once was 8.6.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

Thereafter, 506.1 g of the washed slurry obtained by the procedure 3 wastaken out, and the temperature was adjusted to 40° C. while stirring,and 138.8 g of a 22.1 mass % ammonia aqueous solution and 1171.3 g of2-propanol were added thereto, followed by stirring for 5 minutes, andfurther, 552.8 g of tetraethoxysilane (TEAS) as a silicon compoundhaving a hydrolyzable group was added thereto over minutes. Thereafter,while keeping the liquid temperature at 40° C., stirring was continuedas such for 7 hours to carry out coating with a hydrolysate of thesilicon compound. Thereafter, the obtained solution was washed andsubjected to solid-liquid separation, and the product was collected as acake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1130° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was stirred atabout 60° C. for 24 hours in a 20 mass % NaOH aqueous solution so as tocarry out a treatment of removing the silicon oxide on the surfaces ofthe particles, whereby a slurry containing particles of ε-iron oxide inwhich Fe sites were partially substituted by other metal elements wasobtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less, whereby a washed slurry accordingto Comparative Example 1 was obtained. To the obtained washed slurry, a1 mass % sulfuric acid aqueous solution was added to adjust the pH to6.5 and aggregate substitution-type ε-iron oxide particles, followed byfiltration through a membrane filter, and the solid was collected andthen dried, whereby an iron-based oxide magnetic powder according toComparative Example 1 was obtained. This iron-based oxide magneticpowder had a coercive force of 177 kA/m, a saturation magnetization of17.9 Am²/kg, a square ratio of 0.522, an SFD of 1.00, and an I_(L)/I_(H)value of 0.41.

As a result of subjecting the obtained iron-based oxide magnetic powderaccording to Comparative Example 1 to TEM observation by the sameprocedure as in Example 1, the TEM average particle diameter was 16.6nm, the number ratio of particles with a particle diameter of 8 nm orless was 0.0%, the number ratio of particles with a particle diameter of20 nm or more was 14.9%, the coefficient of variation (CV value) was20%, and the average circularity of the particles was 0.972. In FIG. 5,a TEM photograph of the iron-based oxide magnetic powder obtained inthis Comparative Example is shown. Note that the length indicated by thewhite horizontal line displayed in the lower right of the TEM photographis 100 nm. In addition, a compositional analysis was performed, and themolar compositional ratio of each metal element was calculated when thetotal amount of iron and the substituent metal elements was set to 2mol. The above-mentioned production conditions are shown in Tables 1 and2, and the measurement results for the obtained iron-based oxidemagnetic powders are shown in Tables 3 and 4

Comparative Example 2

In a 5 L reaction vessel, 291.32 g of ferric(III) nitrate nonahydratewith a purity of 99.4%, 80.18 g of a gallium(III) nitrate solution witha Ga concentration of 10.1%, 6.58 g of cobalt(II) nitrate hexahydratewith a purity of 97%, and 7.14 g of titanium(IV) sulfate n-hydrate witha Ti concentration of 14.7% were dissolved in 3214.78 g of pure waterwhile mechanically stirring with a stirring blade in an air atmosphereunder the condition of 40° C. The molar ratio of metal ions in thepreparation solution is as follows: Fe:Ga:Co:Ti=1.635:0.265:0.050:0.050.Note that the number in parentheses after the reagent name indicates thevalence of the metal element.

While mechanically stirring with a stirring blade at 40° C. in an airatmosphere, 166.29 g of a 21.2% ammonia solution was added thereto allat once so that the pH after neutralization became 1.0 or higher and 3.0or lower, and stirring was continued for 2 hours. The mixture was abrown turbid liquid at the beginning of addition, but after 2 hours, theliquid turned into a transparent brown reaction solution, and the pHthereof was 1.96.

Subsequently, 252.66 g of a citric acid solution with a citric acidconcentration of 10 mass % was continuously added thereto over 1 hourunder the condition of 40° C., and thereafter, 200 g of a 10 mass %ammonia solution was added thereto all at once so that the pH became 7.0or higher and 10.0 or lower, thereby adjusting the pH to 8.47, and then,the resulting mixture was held for 1 hour while stirring under thecondition of temperature of 40° C., whereby crystals of an ironoxyhydroxide containing substituent elements of a precursor, which is anintermediate product, were generated (procedure 1). Note that the molarratio with respect to the total amount of the trivalent iron ions andthe ions of the metal M that partially substitutes the Fe sites:O/(Fe+M) is 0.15. The slurry containing the iron oxyhydroxide crystalsobtained by the procedure 1 was dropped onto a collodion film on a gridand adhered thereto, and then, naturally dried. Thereafter, carbon vapordeposition was performed, and the resultant was subjected to TEMobservation. In FIG. 6, a TEM photograph of the precursor particles (theparticles of the substituent element-containing iron oxyhydroxide)obtained in this Comparative Example is shown. Note that the lengthindicated by the white horizontal line displayed in the lower right ofthe TEM photograph is 100 nm. In addition, when the slurry obtained bythe procedure 1 was subjected to measurement of the DLS average diameterof the precursor particles, the result was above the allowablemeasurement range (up to 500 nm), and the DLS average diameter could notbe measured.

Thereafter, while stirring at 30° C. in an air atmosphere, to theprecursor slurry obtained by the procedure 1, 488.13 g oftetraethoxysilane was added over 35 minutes. Stirring was continued assuch for about 1 day while maintaining the temperature at 30° C. tocarry out coating with a silanol derivative generated by hydrolysis.Thereafter, a solution obtained by dissolving 194.7 g of ammoniumsulfate in 300 g of pure water was added thereto, and the resultingsolution was washed and subjected to solid-liquid separation, and theproduct was collected as a cake (procedure 2).

After the precipitate (the precursor coated with SiO₂ in a gel form)obtained by the procedure 2 was dried, the resulting dry powder wassubjected to a heat treatment at 1070° C. for 4 hours in a furnace withan air atmosphere, whereby an iron-based oxide magnetic particle powdercoated with silicon oxide was obtained. Note that the silanol derivativeturns into an oxide when it is subjected to a heat treatment in an airatmosphere (procedure 3).

The heat-treated powder obtained by the procedure 3 was stirred at about70° C. for 24 hours in a 20 mass % NaOH aqueous solution to carry out atreatment of removing the silicon oxide on the surfaces of theparticles. Subsequently, washing was performed until the electricalconductivity of the washed slurry became 15 mS/m or less, and afterdrying, the resultant was subjected to a chemical analysis of thecomposition, XRD measurement, TEM observation, measurement of themagnetic properties, and the like.

The chemical composition of the obtained iron-based oxide magneticparticle powder was substantially the same as the composition at thetime of preparation although the content of Fe slightly increased andthe content of Ga slightly decreased.

As a result of subjecting the iron-based oxide magnetic particle powderobtained in this Comparative Example to TEM observation by the sameprocedure as in Example 1, the average particle diameter was 16.6 nm andthe coefficient of variation (CV value) was 40.2%. In FIG. 7, a TEMphotograph of the iron-based oxide magnetic particle powder obtained inthis Comparative Example is shown. In a differential B-H curve, twopeaks were clearly observed, and the SFD determined based on the halfwidth of the peak of the high Hc component was 0.89, and the ratio ofthe low Hc component I_(L)/I_(H) was 0.50.

Comparative Example 3

In a 5 L reaction vessel, 465.93 g of ferric(III) nitrate nonahydratewith a purity of 99.7%, 152.80 g of a gallium(III) nitrate solution witha Ga concentration of 12.0 mass %, 15.78 g of cobalt(II) nitratehexahydrate with a purity of 97%, and 11.91 g of a titanium(IV) sulfatesolution with a Ti concentration of 15.1 mass % were dissolved in2453.58 g of pure water while mechanically stirring with a stirringblade in an air atmosphere under the condition of 40° C. The molar ratioof metal ions in the preparation solution is as follows: Fe:Ga:Co:Ti1.530:0.350:0.070:0.050.

While mechanically stirring with a stirring blade at 40° C. in an airatmosphere, 268.52 g of a 22.4% ammonia aqueous solution was addedthereto all at once, and stirring was continued for 2 hours (firstneutralization step). The mixture was a brown turbid liquid at thebeginning of addition, but after 2 hours, the liquid turned into atransparent brown reaction solution, and the pH thereof was 1.9.

Subsequently, as a hydroxycarboxylic acid, 288.75 g of a citric acidsolution with a citric acid concentration of 20 mass % was continuouslyadded thereto over 1 hour under the condition of 40° C.(hydroxycarboxylic acid addition step), and thereafter, 152.86 g of a22.4% ammonia solution was added thereto all at once, and then, theresulting mixture was held for 1 hour while stirring under the conditionof temperature of 40° C., whereby crystals of an iron oxyhydroxidecontaining substituent elements of a precursor, which is an intermediateproduct, were generated (procedure 1, second neutralization step). ThepH after adding 152.86 g of the ammonia solution all at once was 8.6.

Thereafter, while stirring at 40° C. in an air atmosphere, to theprecursor slurry obtained by the procedure 1, 416.89 g oftetraethoxysilane (TEOS) was added over 35 minutes. Stirring wascontinued as such for about 1 day to carry out coating with ahydrolysate of the silane compound generated by hydrolysis. Thereafter,the obtained solution was washed and subjected to solid-liquidseparation, and the product was collected as a cake (procedure 2).

After the precipitate (the precursor coated with SiO₂ in a gel form)obtained by the procedure 2 was dried, the resulting dry powder wassubjected to a heat treatment at 971° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of substitution-type ε-iron oxidecoated with silicon oxide was obtained. Note that the hydrolysate of thesilane compound turns into an oxide when it is subjected to a heattreatment in an air atmosphere (procedure 3).

The powder of substitution-type ε-iron oxide coated with silicon oxideobtained by the procedure 3 was stirred at about 60° C. for 24 hours ina 20 mass % NaOH aqueous solution so as to carry out a treatment ofremoving the silicon oxide on the surfaces of the particles, whereby aslurry containing iron-based oxide particles was obtained (procedure 4).

When transmission electron microscope observation was performed for theslurry obtained by the procedure 4, the TEM average particle diameterwas 17.8 nm and the coefficient of variation (CV value) was 39%.

The slurry obtained by the procedure 4 was washed until the electricalconductivity became 15 mS/m or less, whereby a washed slurry accordingto Comparative Example 1 was obtained. To the obtained washed slurry, a1 mass % sulfuric acid aqueous solution was added to adjust the pH to6.5 and aggregate substitution-type ε-iron oxide particles, followed byfiltration through a membrane filter, and the solid was collected andthen dried, whereby an iron-based oxide magnetic powder before aclassification treatment was obtained. This iron-based oxide magneticpowder before classification had a coercive force of 171 (kA/m), asaturation magnetization of 15.7 (Am²/kg), a square ratio of 0.433, anSFD of 1.40, an value of 0.82, and a BET specific surface area of 85.5m²/g.

Subsequently, to the washed slurry, a 25 mass % tetramethylammoniumhydroxide (hereinafter referred to as TMAOH) aqueous solution was addedas a surface modifier, whereby a surface modifier-containing slurryaccording to Comparative Example 3 was obtained. The addition amount ofthe TMAOH aqueous solution was set so that the TMAOH concentration inthe surface modifier-containing slurry became 0.065 mol/kg. In thiscase, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of20,000 rpm for 15 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 33 g of asupernatant containing fine particles was removed and a precipitate wasobtained. The gravitational acceleration in the centrifugation treatmentwas set to 48,000 G. Further, when the average secondary particlediameter (DLS particle diameter) of the iron-based oxide in the surfacemodifier-containing slurry after performing the ultrasonic treatment wasmeasured, it was 29 nm.

Thereafter, for the obtained precipitate, the operation of addition of33 g of a 0.065 mol/kg TMAOH aqueous solution, the above-mentionedultrasonic dispersion treatment, the above-mentioned centrifugationtreatment, and removal of 33 g of a supernatant was repeated 9 times,whereby a slurry of an iron-based oxide magnetic powder according toComparative Example 3 was obtained.

When TEM observation was performed for the obtained slurry of theiron-based oxide magnetic powder according to Comparative Example 3, theTEM average particle diameter was 18.8 nm and the coefficient ofvariation (CV value) was 29%. The TEM observation for the slurry refersto dropping the slurry onto a collodion film on a grid and adhering theslurry thereto, followed by natural drying and then carbon vapordeposition, and subjecting the resultant to TEM observation. In FIG. 1,a TEM photograph of the iron-based oxide magnetic powder obtained inthis Example is shown. Note that the length indicated by the whitevertical line displayed in the center on the left side of the TEMphotograph is 100 nm.

To the obtained slurry of the iron-based oxide magnetic powder accordingto Comparative Example 3, 33 g of pure water was added, and a 1 mass %sulfuric acid aqueous solution was added thereto to adjust the pH to6.5, followed by membrane filtration, and the cake was collected andthen dried, whereby an iron-based oxide magnetic powder according toComparative Example 3 was obtained.

When the chemical composition of the obtained iron-based oxide magneticpowder was calculated so that the sum of the molar ratios of Fe, Ga, Co,and Ti becomes 2.0, it was as shown in Table 1. Further, the value was0.25.

Example 10

As a raw material solution, 1013 g of an aqueous solution with anFe(III) concentration of 1.112 mol/kg and a chloride ion concentrationof 3.337 mol/kg was prepared. Subsequently, in a 5 L reaction vessel,1603 g of an aqueous solution with an Fe(III) concentration of 0.234mol/kg and a chloride ion concentration of 0.703 mol/kg was placed, andthe liquid temperature was adjusted to 30° C. while mechanicallystirring with a stirring blade. Subsequently, while continuing stirringand keeping the liquid temperature at 40° C., the raw material solutionand a 3.337 mol/kg ammonia aqueous solution were simultaneously addedinto the aqueous solution, and thereafter, stirring was performed for 60minutes while keeping the liquid temperature at 30° C., whereby acolloidal solution was obtained (procedure 1, first neutralizationstep). Here, the raw material aqueous solution was continuously addedover 240 minutes at an addition rate of 4.2 g/min, and the ammoniaaqueous solution was continuously added over 240 minutes at an additionrate of 4.2 g/min. While adding the raw material aqueous solution andthe ammonia aqueous solution, the ratio of the cumulative additionamount of the alkali to the total cumulative addition amount of the acidgroup contained in the raw material solution is 1.0 equivalents. The pHof the obtained colloidal solution was 2.3.

Subsequently, to the colloidal solution obtained by the procedure 1, 289g of a citric acid solution with a citric acid concentration of 1.041mol/kg as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 30° C. (hydroxycarboxylic acid additionstep), and thereafter, 189 g of a 12.864 mol/kg ammonia solution wasadded thereto all at once, and then, the resulting mixture was held for10 minutes while stirring under the condition of temperature of 30° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding189 g of the ammonia solution all at once was 9.2.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 127.8 nm.

Subsequently, 857 g of the washed slurry obtained by the procedure 3 wastaken out, and pure water was added thereto to give a total amount of950 g. The temperature was adjusted to 40° C. while stirring, and 1171 gof ethanol was added thereto, followed by stirring for 5 minutes, andthereafter, 260 g of a 21.92 mass % ammonia aqueous solution was addedthereto. Further, 1028 g of tetraethoxysilane (TEAS) as a siliconcompound having a hydrolyzable group was added thereto over 35 minutes.Thereafter, while keeping the liquid temperature at 40° C., stirring wascontinued as such for 1 hour to carry out coating with a hydrolysate ofthe silicon compound. Thereafter, the obtained solution was washed andsubjected to solid-liquid separation, and the product was collected as acake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1130° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 24 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 35 mass % TEAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 5.0% was obtained. Here, the additionamount of the TEAOH aqueous solution was set so that the TEAOHconcentration in the surface modifier-containing slurry became 0.099mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of20,000 rpm for 60 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry containing fine particles was removed and a slurry onthe precipitate side was obtained. The gravitational acceleration in thecentrifugation treatment was set to 48,000 G. For the obtained slurry onthe precipitate side, the operation of addition of 30 g of a 0.099mol/kg TEAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 20,000 rpm and 48,000 G for 60 minutes, and removal of 30 gof a supernatant slurry was repeated further 9 times, whereby a slurryon the precipitate side was obtained.

After 30 g of a 0.099 mol/kg TEAOH aqueous solution was added to 10 g ofthe obtained slurry on the precipitate side, the resulting mixture wassubjected to an ultrasonic dispersion treatment for 1 hour with anultrasonic cleaner (Yamato 5510 Branson, manufactured by (YamatoScientific) Co., Ltd.), and then subjected to a centrifugation treatmentat a rotation speed of 20,000 rpm for 50 minutes with an R20A2 rotor ofa centrifuge (himac CR21GII, manufactured by Hitachi Koki Co., Ltd.),and 30 g of a supernatant slurry containing fine particles was removed,and a slurry on the precipitate side was obtained. The gravitationalacceleration in the centrifugation treatment was set to 48,000 G. Forthe obtained slurry on the precipitate side, the operation of additionof 30 g of a 0.099 mol/kg TEAOH aqueous solution, ultrasonic dispersion,a centrifugation treatment at 20,000 rpm and 48,000 G for 50 minutes,and removal of 30 g of a supernatant slurry was repeated further times,whereby a slurry on the precipitate side was obtained.

After 30 g of a 0.099 mol/kg TEAOH aqueous solution was added to 10 g ofthe obtained slurry on the precipitate side, the resulting mixture wassubjected to an ultrasonic dispersion treatment for 1 hour with anultrasonic cleaner (Yamato 5510 Branson, manufactured by (YamatoScientific) Co., Ltd.), and then subjected to a centrifugation treatmentat a rotation speed of 20,000 rpm for 40 minutes with an R20A2 rotor ofa centrifuge (himac CR21GII, manufactured by Hitachi Koki Co., Ltd.),and 30 g of a supernatant slurry was collected. The gravitationalacceleration in the centrifugation treatment was set to 48,000 G. For anobtained slurry on the precipitate side, the operation of addition of 30g of a 0.099 mol/kg TEAOH aqueous solution, ultrasonic dispersion, acentrifugation treatment at 20,000 rpm and 48,000 G for 40 minutes, andcollection of 30 g of a supernatant slurry was repeated further 9 times,whereby a total of 300 g of the supernatant slurry was collected. To thecollected supernatant slurry, a 1 mass % sulfuric acid aqueous solutionwas added to adjust the pH to 6.5 and aggregate substitution-type ε-ironoxide particles, followed by filtration through a membrane filter, andthe solid was collected and then dried, whereby an iron-based oxidemagnetic powder according to Example 10 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of560 (kA/m), a saturation magnetization of 12.8 (Am²/kg), a square ratioof 0.544, an SFD of 1.99, and an I_(L)/I_(H) value of 0.47. Further, theobtained iron-based oxide magnetic powder was dispersed in pure waterusing an ultrasonic disperser to prepare a slurry, and the preparedslurry was dropped onto a collodion film on a grid and adhered thereto,and then, naturally dried. Thereafter, carbon vapor deposition wasperformed, and the resultant was subjected to TEM observation. As aresult of measuring 200 particles by TEM observation, the TEM averageparticle diameter was 10.3 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 1.2%, the number ratio ofparticles with a particle diameter of 20 nm or more was 0.0%, thecoefficient of variation (CV value) was 9%, and the average circularityof the particles was 0.970. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 11

As a raw material solution, 1019 g of an aqueous solution with anFe(III) concentration of 1.056 mol/kg, Co(II) concentration of 0.066mol/kg, a chloride ion concentration of 3.167 mol/kg, and a nitrate ionconcentration of 0.133 mol/kg was prepared. Subsequently, in a 5 Lreaction vessel, 1598 g of an aqueous solution with an Fe(III)concentration of 0.225 mol/kg and a chloride ion concentration of 0.674mol/kg was placed, and the liquid temperature was adjusted to 40° C.while mechanically stirring with a stirring blade. Subsequently, whilecontinuing stirring and keeping the liquid temperature at 40° C., theraw material solution and a 3.267 mol/kg ammonia aqueous solution weresimultaneously added into the aqueous solution, and thereafter, stirringwas performed for 60 minutes while keeping the liquid temperature at 40°C., whereby a colloidal solution was obtained (procedure 1, firstneutralization step). Here, the raw material aqueous solution wascontinuously added over 240 minutes at an addition rate of 4.2 g/min,and the ammonia aqueous solution was continuously added over 240 minutesat an addition rate of 4.2 g/min. While adding the raw material aqueoussolution and the ammonia aqueous solution, the ratio of the cumulativeaddition amount of the alkali to the total cumulative addition amount ofthe acid group contained in the raw material solution is 0.99equivalents. The pH of the obtained colloidal solution was 2.1.

Subsequently, to the colloidal solution obtained by the procedure 1, 289g of a citric acid solution with a citric acid concentration of 1.041mol/kg as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 194 g of a 12.347 mol/kg ammonia solution wasadded thereto all at once, and then, the resulting mixture was held for10 minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding194 g of the ammonia solution all at once was 9.2.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 147.1 nm.

Subsequently, 894 g of the washed slurry obtained by the procedure 3 wastaken out, and pure water was added thereto to give a total amount of950 g. The temperature was adjusted to 40° C. while stirring, and 1171 gof ethanol was added thereto, followed by stirring for 5 minutes, andthereafter, 271 g of a 21.04 mass % ammonia aqueous solution was addedthereto. Further, 1028 g of tetraethoxysilane (TEOS) as a siliconcompound having a hydrolyzable group was added thereto over 35 minutes.Thereafter, while keeping the liquid temperature at 40° C., stirring wascontinued as such for 1 hour to carry out coating with a hydrolysate ofthe silicon compound. Thereafter, the obtained solution was washed andsubjected to solid-liquid separation, and the product was collected as acake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1130° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 24 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 35 mass % TEAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 3.1% was obtained. Here, the additionamount of the TEAOH aqueous solution was set so that the TEAOHconcentration in the surface modifier-containing slurry became 0.099mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of20,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 48,000 G. For an obtained slurry onthe precipitate side, the operation of addition of 30 g of a 0.099mol/kg TEAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 20,000 rpm and 48,000 G for 30 minutes, and collection of30 g of a supernatant slurry was repeated further 9 times, whereby atotal of 300 g of the supernatant slurry was collected. After 300 g ofthe collected supernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 16,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry wascollected. The gravitational acceleration in the centrifugationtreatment was set to 31,000 G. For an obtained slurry on the precipitateside, the operation of addition of 30 g of a 0.099 mol/kg TEAOH aqueoussolution, ultrasonic dispersion, a centrifugation treatment at 16,000rpm and 31,000 G for 30 minutes, and collection of 30 g of a supernatantslurry was repeated further 9 times, whereby a total of 300 g of thesupernatant slurry was collected. After 300 g of the collectedsupernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Kaki Co., Ltd.), and 30 g of a supernatant slurry containingfine particles was removed and a slurry on the precipitate side wasobtained. The gravitational acceleration in the centrifugation treatmentwas set to 39,000 G. For the slurry on the precipitate side, theoperation of addition of 30 g of a 0.099 mol/kg TEAOH aqueous solution,ultrasonic dispersion, a centrifugation treatment at 18,000 rpm and39,000 G for 30 minutes, and removal of 30 g of a supernatant slurry wasrepeated further times, whereby a slurry on the precipitate side wasobtained. To the obtained slurry on the precipitate side, a 1 mass %sulfuric acid aqueous solution was added to adjust the pH to 6.5 andaggregate substitution-type ε-iron oxide particles, followed byfiltration through a membrane filter, and the solid was collected andthen dried, whereby an iron-based oxide magnetic powder according toExample 11 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of875 (kA/m), a saturation magnetization of 20.6 (Am²/kg), a square ratioof 0.502, an SFD of 1.66, and an I_(L)/I_(H) value of 0.50. Further, theobtained iron-based oxide magnetic powder was dispersed in pure waterusing an ultrasonic disperser to prepare a slurry, and the preparedslurry was dropped onto a collodion film on a grid and adhered thereto,and then, naturally dried. Thereafter, carbon vapor deposition wasperformed, and the resultant was subjected to TEM observation. As aresult of measuring 200 particles by TEM observation, the TEM averageparticle diameter was 16.5 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.5%, the number ratio ofparticles with a particle diameter of 20 nm or more was 3.0%, thecoefficient of variation (CV value) was 12%, and the average circularityof the particles was 0.966. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 12

As a raw material solution, 1021 g of an aqueous solution with anFe(III) concentration of 1.055 mol/kg, a Ti(IV) concentration of 0.066mol/kg, and a chloride ion concentration of 3.332 mol/kg was prepared.Subsequently, in a 5 L reaction vessel, 1598 g of an aqueous solutionwith an Fe(III) concentration of 0.225 mol/kg and a chloride ionconcentration of 0.674 mol/kg was placed, and the liquid temperature wasadjusted to 40° C. while mechanically stirring with a stirring blade.Subsequently, while continuing stirring and keeping the liquidtemperature at 40° C., the raw material solution and a 3.290 mol/kgammonia aqueous solution were simultaneously added into the aqueoussolution, and thereafter, stirring was performed for 60 minutes whilekeeping the liquid temperature at 40° C., whereby a colloidal solutionwas obtained (procedure 1, first neutralization step). Here, the rawmaterial aqueous solution was continuously added over 240 minutes at anaddition rate of 4.3 g/min, and the ammonia aqueous solution wascontinuously added over 240 minutes at an addition rate of 4.3 g/min.While adding the raw material aqueous solution and the ammonia aqueoussolution, the ratio of the cumulative addition amount of the alkali tothe total cumulative addition amount of the acid group contained in theraw material solution is 0.99 equivalents. The pH of the obtainedcolloidal solution was 1.8.

Subsequently, to the colloidal solution obtained by the procedure 1, 289g of a citric acid solution with a citric acid concentration of 1.041mol/kg as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 196 g of a 12.347 mol/kg ammonia solution wasadded thereto all at once, and then, the resulting mixture was held for10 minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding196 g of the ammonia solution all at once was 9.1.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 36.1 nm.

Subsequently, 882 g of the washed slurry obtained by the procedure 3 wastaken out, and pure water was added thereto to give a total amount of950 g. The temperature was adjusted to 40° C. while stirring, and 1171 gof ethanol was added thereto, followed by stirring for 5 minutes, andthereafter, 271 g of a 21.04 mass % ammonia aqueous solution was addedthereto. Further, 1028 g of tetraethoxysilane (TEOS) as a siliconcompound having a hydrolyzable group was added thereto over 35 minutes.Thereafter, while keeping the liquid temperature at 40° C., stirring wascontinued as such for 1 hour to carry out coating with a hydrolysate ofthe silicon compound. Thereafter, the obtained solution was washed andsubjected to solid-liquid separation, and the product was collected as acake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1130° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 15 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 35 mass % TEAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 3.1% was obtained. Here, the additionamount of the TEAOH aqueous solution was set so that the TEAOHconcentration in the surface modifier-containing slurry became 0.099mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of20,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 48,000 G. For an obtained slurry onthe precipitate side, the operation of addition of 30 g of a 0.099mol/kg TEAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 20,000 rpm and 48,000 G for 30 minutes, and collection of30 g of a supernatant slurry was repeated further 9 times, whereby atotal of 300 g of the supernatant slurry was collected. After 300 g ofthe collected supernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 16,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry wascollected. The gravitational acceleration in the centrifugationtreatment was set to 31,000 G. For an obtained slurry on the precipitateside, the operation of addition of 30 g of a 0.099 mol/kg TEAOH aqueoussolution, ultrasonic dispersion, a centrifugation treatment at 16,000rpm and 31,000 G for 30 minutes, and collection of 30 g of a supernatantslurry was repeated further 9 times, whereby a total of 300 g of thesupernatant slurry was collected. After 300 g of the collectedsupernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry containingfine particles was removed and a slurry on the precipitate side wasobtained. The gravitational acceleration in the centrifugation treatmentwas set to 39,000 G. For the slurry on the precipitate side, theoperation of addition of 30 g of a 0.099 mol/kg TEAOH aqueous solution,ultrasonic dispersion, a centrifugation treatment at 18,000 rpm and39,000 G for 30 minutes, and removal of 30 g of a supernatant slurry wasrepeated further times, whereby a slurry on the precipitate side wasobtained. To the obtained slurry on the precipitate side, a 1 mass %sulfuric acid aqueous solution was added to adjust the pH to 6.5 andaggregate substitution-type ε-iron oxide particles, followed byfiltration through a membrane filter, and the solid was collected andthen dried, whereby an iron-based oxide magnetic powder according toExample 12 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of1267 (kA/m), a saturation magnetization of 16.0 (Am²/kg), a square ratioof 0.546, an SFD of 0.50, and an I_(L)/IN value of 0.12. Further, theobtained iron-based oxide magnetic powder was dispersed in pure waterusing an ultrasonic disperser to prepare a slurry, and the preparedslurry was dropped onto a collodion film on a grid and adhered thereto,and then, naturally dried. Thereafter, carbon vapor deposition wasperformed, and the resultant was subjected to TEM observation. As aresult of measuring 200 particles by TEM observation, the TEM averageparticle diameter was 15.9 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.0%, the number ratio ofparticles with a particle diameter of 20 nm or more was 1.6%, thecoefficient of variation (CV value) was 12%, and the average circularityof the particles was 0.974. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 13

As a raw material solution, 1018 g of an aqueous solution with anFe(III) concentration of 1.045 mol/kg, a Co(II) concentration of 0.050mol/kg, a Ti(IV) concentration of 0.033 mol/kg, a chloride ionconcentration of 3.219 mol/kg, and a nitrate ion concentration of 0.100mol/kg was prepared. Subsequently, in a 5 L reaction vessel, 1597 g ofan aqueous solution with an Fe(III) concentration of 0.222 mol/kg and achloride ion concentration of 0.666 mol/kg was placed, and the liquidtemperature was adjusted to 40° C. while mechanically stirring with astirring blade. Subsequently, while continuing stirring and keeping theliquid temperature at 40° C., the raw material solution and a 3.272mol/kg ammonia aqueous solution were simultaneously added into theaqueous solution, and thereafter, stirring was performed for 60 minuteswhile keeping the liquid temperature at 40° C., whereby a colloidalsolution was obtained (procedure 1, first neutralization step). Here,the raw material aqueous solution was continuously added over 240minutes at an addition rate of 4.2 g/min, and the ammonia aqueoussolution was continuously added over 240 minutes at an addition rate of4.2 g/min. While adding the raw material aqueous solution and theammonia aqueous solution, the ratio of the cumulative addition amount ofthe alkali to the total cumulative addition amount of the acid groupcontained in the raw material solution is 0.99 equivalents. The pH ofthe obtained colloidal solution was 1.6.

Subsequently, to the colloidal solution obtained by the procedure 1, 289g of a citric acid solution with a citric acid concentration of 1.041mol/kg as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 196 g of a 12.512 mol/kg ammonia solution wasadded thereto all at once, and then, the resulting mixture was held for10 minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding196 g of the ammonia solution all at once was 8.8.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 38.7 nm.

Subsequently, 852 g of the washed slurry obtained by the procedure 3 wastaken out, and pure water was added thereto to give a total amount of950 g. The temperature was adjusted to 40° C. while stirring, and 1171 gof ethanol was added thereto, followed by stirring for 5 minutes, andthereafter, 268 g of a 21.32 mass % ammonia aqueous solution was addedthereto. Further, 1028 g of tetraethoxysilane (TEOS) as a siliconcompound having a hydrolyzable group was added thereto over 35 minutes.Thereafter, while keeping the liquid temperature at 40° C., stirring wascontinued as such for 1 hour to carry out coating with a hydrolysate ofthe silicon compound. Thereafter, the obtained solution was washed andsubjected to solid-liquid separation, and the product was collected as acake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1130° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 24 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements The slurry obtained by the procedure6 was washed until the electrical conductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 35 mass % TEAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 5.7% was obtained. Here, the additionamount of the TEAOH aqueous solution was set so that the TEAOHconcentration in the surface modifier-containing slurry became 0.099mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of16,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 31,000 G. For an obtained slurry onthe precipitate side, the operation of addition of 30 g of a 0.099mol/kg TEAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 16,000 rpm and 31,000 G for 30 minutes, and collection of30 g of a supernatant slurry was repeated further 9 times, whereby atotal of 300 g of the supernatant slurry was collected. After 300 g ofthe collected supernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry containingfine particles was removed and a slurry on the precipitate side wasobtained. The gravitational acceleration in the centrifugation treatmentwas set to 39,000 G. For the obtained slurry on the precipitate side,the operation of addition of 30 g of a 0.099 mol/kg TEAOH aqueoussolution, ultrasonic dispersion, a centrifugation treatment at 18,000rpm and 39,000 G for 30 minutes, and removal of 30 g of a supernatantslurry was repeated further 9 times, whereby a slurry on the precipitateside was obtained. To the obtained slurry on the precipitate side, a 1mass % sulfuric acid aqueous solution was added to adjust the pH to 6.5and aggregate substitution-type ε-iron oxide particles, followed byfiltration through a membrane filter, and the solid was collected andthen dried, whereby an iron-based oxide magnetic powder according toExample 13 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of499 (kA/m), a saturation magnetization of 14.8 (Am²/kg), a square ratioof 0.529, an SFD of 1.78, and an I_(L)/I_(H) value of 0.46. Further, theobtained iron-based oxide magnetic powder was dispersed in pure waterusing an ultrasonic disperser to prepare a slurry, and the preparedslurry was dropped onto a collodion film on a grid and adhered thereto,and then, naturally dried. Thereafter, carbon vapor deposition wasperformed, and the resultant was subjected to TEM observation. As aresult of measuring 200 particles by TEM observation, the TEM averageparticle diameter was 13.3 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 1.4%, the number ratio ofparticles with a particle diameter of 20 nm or more was 0.5%, thecoefficient of variation (CV value) was 17%, and the average circularityof the particles was 0.970. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 14

As a raw material solution, 936 g of an aqueous solution with an Fe(III)concentration of 0.921 mol/kg, a Ga(III) concentration of 0.321 mol/kg,a Co(II) concentration of 0.056 mol/kg, a chloride ion concentration of1.288 mol/kg, and a nitrate ion concentration of 2.659 mol/kg wasprepared. Subsequently, in a 5 L reaction vessel, 1578 g of an aqueoussolution with an Fe(III) concentration of 0.182 mol/kg and a chlorideion concentration of 0.546 mol/kg was placed, and the liquid temperaturewas adjusted to 40° C. while mechanically stirring with a stirringblade. Subsequently, while continuing stirring and keeping the liquidtemperature at 40° C., the raw material solution and a 3.651 mol/kgammonia aqueous solution were simultaneously added into the aqueoussolution, and thereafter, stirring was performed for 60 minutes whilekeeping the liquid temperature at 40° C., whereby a colloidal solutionwas obtained (procedure 1, first neutralization step). Here, the rawmaterial aqueous solution was continuously added over 240 minutes at anaddition rate of 3.9 g/min, and the ammonia aqueous solution wascontinuously added over 240 minutes at an addition rate of 3.9 g/min.While adding the raw material aqueous solution and the ammonia aqueoussolution, the ratio of the cumulative addition amount of the alkali tothe total cumulative addition amount of the acid group contained in theraw material solution is 0.93 equivalents. The pH of the obtainedcolloidal solution was 2.0.

Subsequently, to the colloidal solution obtained by the procedure 1, 289g of a citric acid solution with a citric acid concentration of 1.041mol/kg as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 196 g of a 12.553 mol/kg ammonia solution wasadded thereto all at once, and then, the resulting mixture was held for10 minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding196 g of the ammonia solution all at once was 8.6.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 150 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 195.5 nm.

Subsequently, 852 g of the washed slurry obtained by the procedure 3 wastaken out, and pure water was added thereto to give a total amount of950 g. The temperature was adjusted to 40° C. while stirring, and 1171 gof ethanol was added thereto, followed by stirring for 5 minutes, andthereafter, 268 g of a 21.32 mass % ammonia aqueous solution was addedthereto. Further, 1028 g of tetraethoxysilane (TEOS) as a siliconcompound having a hydrolyzable group was added thereto over 35 minutes.Thereafter, while keeping the liquid temperature at 40° C., stirring wascontinued as such for 1 hour to carry out coating with a hydrolysate ofthe silicon compound. Thereafter, the obtained solution was washed andsubjected to solid-liquid separation, and the product was collected as acake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1060° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 14 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 35 mass % TEAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 3.1% was obtained. Here, the additionamount of the TEAOH aqueous solution was set so that the TEAOHconcentration in the surface modifier-containing slurry became 0.099mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of20,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 48,000 G. For an obtained slurry onthe precipitate side, the operation of addition of 30 g of a 0.099mol/kg TEAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 20,000 rpm and 48,000 G for 30 minutes, and collection of30 g of a supernatant slurry was repeated further 9 times, whereby atotal of 300 g of the supernatant slurry was collected. After 300 g ofthe collected supernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 16,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry wascollected. The gravitational acceleration in the centrifugationtreatment was set to 31,000 G. For an obtained slurry on the precipitateside, the operation of addition of 30 g of a 0.099 mol/kg TEAOH aqueoussolution, ultrasonic dispersion, a centrifugation treatment at 16,000rpm and 31,000 G for 30 minutes, and collection of 30 g of a supernatantslurry was repeated further 9 times, whereby a total of 300 g of thesupernatant slurry was collected. After 300 g of the collectedsupernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry containingfine particles was removed and a slurry on the precipitate side wasobtained. The gravitational acceleration in the centrifugation treatmentwas set to 39,000 G. For the obtained slurry on the precipitate side,the operation of addition of 30 g of a 0.099 mol/kg TEAOH aqueoussolution, ultrasonic dispersion, a centrifugation treatment at 18,000rpm and 39,000 G for 30 minutes, and removal of 30 g of a supernatantslurry was repeated further 9 times, whereby a slurry on the precipitateside was obtained. To the obtained slurry on the precipitate side, a 1mass % sulfuric acid aqueous solution was added to adjust the pH to 6.5and aggregate substitution-type ε-iron oxide particles, followed byfiltration through a membrane filter, and the solid was collected andthen dried, whereby an iron-based oxide magnetic powder according toExample 14 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of324 (kA/m), a saturation magnetization of 16.9 (Am²/kg), a square ratioof 0.626, an SFD of 0.77, and an I_(L)/I_(H) value of 0.20. Further, theobtained iron-based oxide magnetic powder was dispersed in pure waterusing an ultrasonic disperser to prepare a slurry, and the preparedslurry was dropped onto a collodion film on a grid and adhered thereto,and then, naturally dried. Thereafter, carbon vapor deposition wasperformed, and the resultant was subjected to TEM observation. As aresult of measuring 200 particles by TEM observation, the TEM averageparticle diameter was 16.4 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.0%, the number ratio ofparticles with a particle diameter of 20 nm or more was 2.4%, thecoefficient of variation (CV value) was 12%, and the average circularityof the particles was 0.975. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 15

As a raw material solution, 893 g of an aqueous solution with an Fe(III)concentration of 0.993 mol/kg, a Ga(III) concentration of 0.235 mol/kg,a Co(II) concentration of 0.038 mol/kg, a Ni(II) concentration of 0.084mol/kg, a chloride ion concentration of 2.980 mol/kg, and a nitrate ionconcentration of 1.005 mol/kg was prepared. Subsequently, in a 5 Lreaction vessel, 1581 g of an aqueous solution with an Fe(III)concentration of 0.187 mol/kg and a chloride ion concentration of 0.561mol/kg was placed, and the liquid temperature was adjusted to 40° C.while mechanically stirring with a stirring blade. Subsequently, whilecontinuing stirring and keeping the liquid temperature at 40° C., theraw material solution and a 3.734 mol/kg ammonia aqueous solution weresimultaneously added into the aqueous solution, and thereafter, stirringwas performed for 60 minutes while keeping the liquid temperature at 40°C., whereby a colloidal solution was obtained (procedure 1, firstneutralization step). Here, the raw material aqueous solution wascontinuously added over 240 minutes at an addition rate of 3.7 g/min,and the ammonia aqueous solution was continuously added over 240 minutesat an addition rate of 3.7 g/min. While adding the raw material aqueoussolution and the ammonia aqueous solution, the ratio of the cumulativeaddition amount of the alkali to the total cumulative addition amount ofthe acid group contained in the raw material solution is 0.94equivalents. The pH of the obtained colloidal solution was 2.1.

Subsequently, to the colloidal solution obtained by the procedure 1,274.3 g of a citric acid solution with a citric acid concentration of1.041 mol/kg as hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 193 g of a 12.465 mol/kg ammonia solution wasadded thereto all at once, and then, the resulting mixture was held for10 minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding193 g of the ammonia solution all at once was 8.8.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 39.9 nm.

Subsequently, 1057 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 1150 g. The temperature was adjusted to 40° C. while stirring, and1406 g of ethanol was added thereto, followed by stirring for 5 minutes,and thereafter, 347 g of a 21.24 mass % ammonia aqueous solution wasadded thereto. Further, 1327 g of tetraethoxysilane (TEOS) as a siliconcompound having a hydrolyzable group was added thereto over 35 minutes.Thereafter, while keeping the liquid temperature at 40° C., stirring wascontinued as such for 1 hour to carry out coating with a hydrolysate ofthe silicon compound. Thereafter, the obtained solution was washed andsubjected to solid-liquid separation, and the product was collected as acake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1130° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 14 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 35 mass % TEAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 3.1% was obtained. Here, the additionamount of the TEAOH aqueous solution was set so that the TEAOHconcentration in the surface modifier-containing slurry became 0.099mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of20,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 48,000 G. For an obtained slurry onthe precipitate side, the operation of addition of 30 g of a 0.099mol/kg TEAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 20,000 rpm and 48,000 G for 30 minutes, and collection of30 g of a supernatant slurry was repeated further 9 times, whereby atotal of 300 g of the supernatant slurry was collected. After 300 g ofthe collected supernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 16,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry wascollected. The gravitational acceleration in the centrifugationtreatment was set to 31,000 G. For an obtained slurry on the precipitateside, the operation of addition of 30 g of a 0.099 mol/kg TEAOH aqueoussolution, ultrasonic dispersion, a centrifugation treatment at 16,000rpm and 31,000 G for 30 minutes, and collection of 30 g of a supernatantslurry was repeated further 9 times, whereby a total of 300 g of thesupernatant slurry was collected. After 300 g of the collectedsupernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Koki Co., Ltd.), and 30 g of a supernatant slurry containingfine particles was removed and a slurry on the precipitate side wasobtained. The gravitational acceleration in the centrifugation treatmentwas set to 39,000 G. For the obtained slurry on the precipitate side,the operation of addition of 30 g of a 0.099 mol/kg TEAOH aqueoussolution, ultrasonic dispersion, a centrifugation treatment at 18,000rpm and 39,000 G for 30 minutes, and removal of 30 g of a supernatantslurry was repeated further 9 times, whereby a slurry on the precipitateside was obtained. To the obtained slurry on the precipitate side, a 1mass % sulfuric acid aqueous solution was added to adjust the pH to 6.5and aggregate substitution-type ε-iron oxide particles, followed byfiltration through a membrane filter, and the solid was collected andthen dried, whereby an iron-based oxide magnetic powder according toExample 15 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of505 (kA/m), a saturation magnetization of 22.4 (Am²/kg), a square ratioof 0.494, an SFD of 0.72, and an I_(L)/I_(H) value of 0.59. Further, theobtained iron-based oxide magnetic powder was dispersed in pure waterusing an ultrasonic disperser to prepare a slurry, and the preparedslurry was dropped onto a collodion film on a grid and adhered thereto,and then, naturally dried. Thereafter, carbon vapor deposition wasperformed, and the resultant was subjected to TEM observation. As aresult of measuring 200 particles by TEM observation, the TEM averageparticle diameter was 15.8 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.6%, the number ratio ofparticles with a particle diameter of 20 nm or more was 1.8%, thecoefficient of variation (CV value) was 12%, and the average circularityof the particles was 0.973. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 16

As a raw material solution, 895 g of an aqueous solution with an Fe(III)concentration of 0.991 mol/kg, a Ga(III) concentration of 0.235 mol/kg,a Co(II) concentration of 0.038 mol/kg, a Cr(III) concentration of 0.084mol/kg, a chloride ion concentration of 2.974 mol/kg, and a nitrate ionconcentration of 1.087 mol/kg was prepared. Subsequently, in a 5 Lreaction vessel, 1581 g of an aqueous solution with an Fe(III)concentration of 0.187 mol/kg and a chloride ion concentration of 0.561mol/kg was placed, and the liquid temperature was adjusted to 40° C.while mechanically stirring with a stirring blade. Subsequently, whilecontinuing stirring and keeping the liquid temperature at 40° C., theraw material solution and a 3.808 mol/kg ammonia aqueous solution weresimultaneously added into the aqueous solution, and thereafter, stirringwas performed for 60 minutes while keeping the liquid temperature at 40°C., whereby a colloidal solution was obtained (procedure 1, firstneutralization step). Here, the raw material aqueous solution wascontinuously added over 240 minutes at an addition rate of 3.7 g/min,and the ammonia aqueous solution was continuously added over 240 minutesat an addition rate of 3.7 g/min. While adding the raw material aqueoussolution and the ammonia aqueous solution, the ratio of the cumulativeaddition amount of the alkali to the total cumulative addition amount ofthe acid group contained in the raw material solution is 0.94equivalents. The pH of the obtained colloidal solution was 2.0.

Subsequently, to the colloidal solution obtained by the procedure 1,288.8 g of a citric acid solution with a citric acid concentration of1.041 mol/kg as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 196 g of a 12.553 mol/kg ammonia solution wasadded thereto all at once, and then, the resulting mixture was held for10 minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding196 g of the ammonia solution all at once was 8.5.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 354.1 nm.

Subsequently, 1084 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 1150 g. The temperature was adjusted to 40° C. while stirring, and1406 g of ethanol was added thereto, followed by stirring for 5 minutes,and thereafter, 345 g of a 21.39 mass % ammonia aqueous solution wasadded thereto. Further, 1327 g of tetraethoxysilane (TEOS) as a siliconcompound having a hydrolyzable group was added thereto over 35 minutes.Thereafter, while keeping the liquid temperature at 40° C., stirring wascontinued as such for 1 hour to carry out coating with a hydrolysate ofthe silicon compound. Thereafter, the obtained solution was washed andsubjected to solid-liquid separation, and the product was collected as acake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1130° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 60° C. andstirred for 14 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 15 mS/m or less.

Subsequently, to the washed slurry, a 35 mass % TEACH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 3.2% was obtained. Here, the additionamount of the TEACH aqueous solution was set so that the TEACHconcentration in the surface modifier-containing slurry became 0.099mol/kg. In this case, the pH of the slurry became 13.

40 g of the obtained surface modifier-containing slurry was subjected toan ultrasonic dispersion treatment for 1 hour with an ultrasonic cleaner(Yamato 5510 Branson, manufactured by (Yamato Scientific) Co., Ltd.),and then subjected to a centrifugation treatment at a rotation speed of20,000 rpm for 30 minutes with an R20A2 rotor of a centrifuge (himacCR21GII, manufactured by Hitachi Koki Co., Ltd.), and 30 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 48,000 G. For an obtained slurry onthe precipitate side, the operation of addition of 30 g of a 0.099mol/kg TEACH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 20,000 rpm and 48,000 G for 30 minutes, and collection of30 g of a supernatant slurry was repeated further 9 times, whereby atotal of 300 g of the supernatant slurry was collected. After 300 g ofthe collected supernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 16,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Kaki Co., Ltd.), and 30 g of a supernatant slurry wascollected. The gravitational acceleration in the centrifugationtreatment was set to 31,000 G. For an obtained slurry on the precipitateside, the operation of addition of 30 g of a 0.099 mol/kg TEAOH aqueoussolution, ultrasonic dispersion, a centrifugation treatment at 16,000rpm and 31,000 G for 30 minutes, and collection of 30 g of a supernatantslurry was repeated further 9 times, whereby a total of 300 g of thesupernatant slurry was collected. After 300 g of the collectedsupernatant slurry was mixed, 40 g thereof was taken out.

40 g of the taken out supernatant slurry was subjected to acentrifugation treatment at a rotation speed of 18,000 rpm for 30minutes with an R20A2 rotor of a centrifuge (himac CR21GII, manufacturedby Hitachi Kaki Co., Ltd.), and 30 g of a supernatant slurry containingfine particles was removed and a slurry on the precipitate side wasobtained. The gravitational acceleration in the centrifugation treatmentwas set to 39,000 G. For the obtained slurry on the precipitate side,the operation of addition of 30 g of a 0.099 mol/kg TEACH aqueoussolution, ultrasonic dispersion, a centrifugation treatment at 18,000rpm and 39,000 G for 30 minutes, and removal of 30 g of a supernatantslurry was repeated further 9 times, whereby a slurry on the precipitateside was obtained. To the obtained slurry on the precipitate side, a 1mass % sulfuric acid aqueous solution was added to adjust the pH to 6.5and aggregate substitution-type ε-iron oxide particles, followed byfiltration through a membrane filter, and the solid was collected andthen dried, whereby an iron-based oxide magnetic powder according toExample 16 was obtained.

The obtained iron-based oxide magnetic powder had a coercive force of471 (kA/m), a saturation magnetization of 16.1 (Am²/kg), a square ratioof 0.542, an SFD of 0.78, and an I_(L)/I_(H) value of 0.18. Further, theobtained iron-based oxide magnetic powder was dispersed in pure waterusing an ultrasonic disperser to prepare a slurry, and the preparedslurry was dropped onto a collodion film on a grid and adhered thereto,and then, naturally dried. Thereafter, carbon vapor deposition wasperformed, and the resultant was subjected to TEM observation. As aresult of measuring 200 particles by TEM observation, the TEM averageparticle diameter was 15.8 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.0%, the number ratio ofparticles with a particle diameter of 20 nm or more was 2.0%, thecoefficient of variation (CV value) was 13%, and the average circularityof the particles was 0.975. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 17

As a raw material solution, 1829 g of an aqueous solution with anFe(III) concentration of 1.257 mol/kg, a Ga(III) concentration of 0.288mol/kg, a Co(II) concentration of 0.058 mol/kg, a Ti(IV) concentrationof 0.041 mol/kg, a chloride ion concentration of 3.876 mol/kg, and anitrate ion concentration of 1.075 mol/kg was prepared. Subsequently, ina 10 L reaction vessel, 3000 g of pure water was placed, and the liquidtemperature was adjusted to 40° C. while mechanically stirring with astirring blade. Subsequently, while continuing stirring and keeping theliquid temperature at 40° C., the raw material solution and a 3.961mol/kg ammonia aqueous solution were simultaneously added into the purewater, and thereafter, stirring was performed for 60 minutes whilekeeping the liquid temperature at 40° C., whereby a colloidal solutionwas obtained (procedure 1, first neutralization step). Here, the rawmaterial aqueous solution was continuously added over 240 minutes at anaddition rate of 7.6 g/min, and the ammonia aqueous solution wascontinuously added over 240 minutes at an addition rate of 7.6 g/min.While adding the raw material aqueous solution and the ammonia aqueoussolution, the ratio of the cumulative addition amount of the alkali tothe total cumulative addition amount of the acid group contained in theraw material solution is 0.80 equivalents. The pH of the obtainedcolloidal solution was 1.8.

Subsequently, to the colloidal solution obtained by the procedure 1, 578g of a citric acid solution with a citric acid concentration of 1.041mol/kg as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 377 g of a 12.987 mol/kg ammonia solution wasadded thereto all at once, and then, the resulting mixture was held for10 minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding377 g of the ammonia solution all at once was 8.8.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 1 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 46.2 nm.

Subsequently, 1294 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 2400 g. The temperature was adjusted to 40° C. while stirring, and2928 g of ethanol was added thereto, followed by stirring for 5 minutes,and thereafter, 694 g of a 22.13 mass % ammonia aqueous solution wasadded thereto. Further, 2764 g of tetraethoxysilane (TEOS) as a siliconcompound having a hydrolyzable group was added thereto over 35 minutes.Thereafter, while keeping the liquid temperature at 40° C., stirring wascontinued as such for 1 hour to carry out coating with a hydrolysate ofthe silicon compound. Thereafter, the obtained solution was washed andsubjected to solid-liquid separation, and the product was collected as acake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1116° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 80° C. andstirred for 20 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 1 mS/m or less.

Subsequently, to the washed slurry, a 25 mass % TMAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 36.0% was obtained. Here, the additionamount of the TMAOH aqueous solution was set so that the TMAOHconcentration in the surface modifier-containing slurry became 0.065mol/kg. In this case, the pH of the slurry became 13.

460 g of the obtained surface modifier-containing slurry was subjectedto an ultrasonic dispersion treatment for 1 hour with an ultrasoniccleaner (Yamato 5510 Branson, manufactured by (Yamato Scientific) Co.,Ltd.), and then subjected to a centrifugation treatment at a rotationspeed of 8800 rpm for 130 minutes with an R10A3 rotor of a centrifuge(himac CR21GII, manufactured by Hitachi Koki Co., Ltd.), and 250 g of asupernatant slurry containing fine particles was removed and a slurry onthe precipitate side was obtained. The gravitational acceleration in thecentrifugation treatment was set to 15,000 G. For the obtained slurry onthe precipitate side, the operation of addition of 250 g of a 0.065mol/kg TMAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 8800 rpm and 15,000 G for 130 minutes, and removal of 250 gof a supernatant slurry was repeated further 9 times, whereby a slurryon the precipitate side was obtained.

After 250 g of a 0.065 mol/kg TMAOH aqueous solution was added to 210 gof the obtained slurry on the precipitate side, the resulting mixturewas subjected to an ultrasonic dispersion treatment for 1 hour with anultrasonic cleaner (Yamato 5510 Branson, manufactured by (YamatoScientific) Co., Ltd.), and then subjected to a centrifugation treatmentat a rotation speed of 8800 rpm for 70 minutes with an R10A3 rotor of acentrifuge (himac CR21GII, manufactured by Hitachi Koki Co., Ltd.), and250 g of a supernatant slurry was collected. The gravitationalacceleration in the centrifugation treatment was set to 15,000 G. For anobtained slurry on the precipitate side, the operation of addition of250 g of a 0.065 mol/kg TMAOH aqueous solution, ultrasonic dispersion, acentrifugation treatment at 8800 rpm and 15,000 G for 70 minutes, andcollection of 250 g of a supernatant slurry was repeated further 15times, whereby a total of 4000 g of the supernatant slurry wascollected. To the collected supernatant slurry, a 1 mass % sulfuric acidaqueous solution was added to adjust the pH to 6.5 and aggregatesubstitution-type ε-iron oxide particles, followed by filtration througha membrane filter, and the solid was collected and then dried, wherebyan iron-based oxide magnetic powder according to Example 17 wasobtained.

The obtained iron-based oxide magnetic powder had a coercive force of171 (kA/m), a saturation magnetization of 16.6 (Am²/kg), a square ratioof 0.556, an SFD of 1.21, and an I_(L)/I_(H) value of 0.33. Further, theobtained iron-based oxide magnetic powder was dispersed in pure waterusing an ultrasonic disperser to prepare a slurry, and the preparedslurry was dropped onto a collodion film on a grid and adhered thereto,and then, naturally dried. Thereafter, carbon vapor deposition wasperformed, and the resultant was subjected to TEM observation. As aresult of measuring 200 particles by TEM observation, the TEM averageparticle diameter was 15.3 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.3%, the number ratio ofparticles with a particle diameter of 20 nm or more was 2.1%, thecoefficient of variation (CV value) was 14%, and the average circularityof the particles was 0.973. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 18

As a raw material solution, 2687 g of an aqueous solution with anFe(III) concentration of 1.430 mol/kg, a Ga(III) concentration of 0.147mol/kg, a Co(II) concentration of 0.059 mol/kg, a Ti(IV) concentrationof 0.042 mol/kg, a chloride ion concentration of 4.398 mol/kg, and anitrate ion concentration of 0.592 mol/kg was prepared. Subsequently, ina 10 L reaction vessel, 4500 g of pure water was placed, and the liquidtemperature was adjusted to 40° C. while mechanically stirring with astirring blade. Subsequently, while continuing stirring and keeping theliquid temperature at 40° C., the raw material solution and a 3.742mol/kg ammonia aqueous solution were simultaneously added into the purewater, and thereafter, stirring was performed for 60 minutes whilekeeping the liquid temperature at 40° C., whereby a colloidal solutionwas obtained (procedure 1, first neutralization step). Here, the rawmaterial aqueous solution was continuously added over 240 minutes at anaddition rate of 11.2 g/min, and the ammonia aqueous solution wascontinuously added over 240 minutes at an addition rate of 11.2 g/min.While adding the raw material aqueous solution and the ammonia aqueoussolution, the ratio of the cumulative addition amount of the alkali tothe total cumulative addition amount of the acid group contained in theraw material solution is 0.75 equivalents. The pH of the obtainedcolloidal solution was 1.7.

Subsequently, to the colloidal solution obtained by the procedure 1, 866g of a citric acid solution with a citric acid concentration of 1.041mol/kg as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 557 g of a 12.993 mol/kg ammonia solution wasadded thereto all at once, and then, the resulting mixture was held for10 minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding557 g of the ammonia solution all at once was 8.5.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 30 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 37.8 nm.

Subsequently, 710 g of the washed slurry obtained by the procedure 3 wastaken out, and pure water was added thereto to give a total amount of1650 g. The temperature was adjusted to 40° C. while stirring, and 2050g of ethanol was added thereto, followed by stirring for 5 minutes, andthereafter, 971 g of a 22.14 mass % ammonia aqueous solution was addedthereto. Further, 3870 g of tetraethoxysilane (TEOS) as a siliconcompound having a hydrolyzable group was added thereto over 35 minutes.Thereafter, while keeping the liquid temperature at 40° C., stirring wascontinued as such for 1 hour to carry out coating with a hydrolysate ofthe silicon compound. Thereafter, the obtained solution was washed andsubjected to solid-liquid separation, and the product was collected as acake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1115° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 80° C. andstirred for 20 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 1 mS/m or less.

Subsequently, to the washed slurry, a 25 mass % TMAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 40.0% was obtained. Here, the additionamount of the TMAOH aqueous solution was set so that the TMAOHconcentration in the surface modifier-containing slurry became 0.065mol/kg. In this case, the pH of the slurry became 13.

460 g of the obtained surface modifier-containing slurry was subjectedto an ultrasonic dispersion treatment for 1 hour with an ultrasoniccleaner (Yamato 5510 Branson, manufactured by (Yamato Scientific) Co.,Ltd.), and then subjected to a centrifugation treatment at a rotationspeed of 8800 rpm for 60 minutes with an R10A3 rotor of a centrifuge(himac CR21GII, manufactured by Hitachi Koki Co., Ltd.), and 250 g of asupernatant slurry was collected. The gravitational acceleration in thecentrifugation treatment was set to 15,000 G. For an obtained slurry onthe precipitate side, the operation of addition of 250 g of a 0.065mol/kg TMAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 8800 rpm and 15,000 G for 70 minutes, and collection of 250g of a supernatant slurry was repeated further 12 times, whereby a totalof 3250 g of the supernatant slurry was collected. After 3250 g of thecollected supernatant slurry was mixed, 460 g thereof was taken out.

460 g of the taken out supernatant slurry was subjected to an ultrasonicdispersion treatment for 1 hour with an ultrasonic cleaner (Yamato 5510Branson, manufactured by (Yamato Scientific) Co., Ltd.), and thensubjected to a centrifugation treatment at a rotation speed of 8800 rpmfor 130 minutes with an R10A3 rotor of a centrifuge (himac CR21GII,manufactured by Hitachi Koki Co., Ltd.), and 250 g of a supernatantslurry containing fine particles was removed and a slurry on theprecipitate side was obtained. The gravitational acceleration in thecentrifugation treatment was set to 15,000 G. For the obtained slurry onthe precipitate side, the operation of addition of 250 g of a 0.065mol/kg TMAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 8800 rpm and 15,000 G for 130 minutes, and removal of 250 gof a supernatant slurry was repeated further 8 times, whereby a slurryon the precipitate side was obtained.

After 250 g of a 0.065 mol/kg TMAOH aqueous solution was added to 210 gof the obtained slurry on the precipitate side, the resulting mixturewas subjected to an ultrasonic dispersion treatment for 1 hour with anultrasonic cleaner (Yamato 5510 Branson, manufactured by (YamatoScientific) Co., Ltd.), and then subjected to a centrifugation treatmentat a rotation speed of 8800 rpm for 70 minutes with an R10A3 rotor of acentrifuge (himac CR21GII, manufactured by Hitachi Koki Co., Ltd.), and250 g of a supernatant slurry was collected. The gravitationalacceleration in the centrifugation treatment was set to 15,000 G. For anobtained slurry on the precipitate side, the operation of addition of250 g of a 0.065 mol/kg TMAOH aqueous solution, ultrasonic dispersion, acentrifugation treatment at 8800 rpm and 15,000 G for 70 minutes, andcollection of 250 g of a supernatant slurry was repeated further 12times, whereby a total of 3250 g of the supernatant slurry wascollected. To the collected supernatant slurry, a 1 mass % sulfuric acidaqueous solution was added to adjust the pH to 6.5 and aggregatesubstitution-type ε-iron oxide particles, followed by filtration througha membrane filter, and the solid was collected and then dried, wherebyan iron-based oxide magnetic powder according to Example 18 wasobtained.

The obtained iron-based oxide magnetic powder had a coercive force of349 (kA/m), a saturation magnetization of 15.3 (Am²/kg), a square ratioof 0.643, an SFS of 0.71, and an I_(L)/I_(H) value of 0.15. Further, theobtained iron-based oxide magnetic powder was dispersed in pure waterusing an ultrasonic disperser to prepare a slurry, and the preparedslurry was dropped onto a collodion film on a grid and adhered thereto,and then, naturally dried. Thereafter, carbon vapor deposition wasperformed, and the resultant was subjected to TEM observation. As aresult of measuring 200 particles by TEM observation, the TEM averageparticle diameter was 15.8 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.0%, the number ratio ofparticles with a particle diameter of 20 nm or more was 1.4%, thecoefficient of variation (CV value) was 12%, and the average circularityof the particles was 0.975. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 19

As a raw material solution, 2586 g of an aqueous solution with anFe(III) concentration of 1.000 mol/kg, a Ga(III) concentration of 0.349mol/kg, a Co(II) concentration of 0.061 mol/kg, a chloride ionconcentration of 3.001 mol/kg, and a nitrate ion concentration of 1.199mol/kg was prepared. Subsequently, in a 10 L reaction vessel, 4735 g ofan aqueous solution with an Fe(III) concentration of 0.182 mol/kg and achloride ion concentration of 0.546 mol/kg was placed, and the liquidtemperature was adjusted to 40° C. while mechanically stirring with astirring blade. Subsequently, while continuing stirring and keeping theliquid temperature at 40° C., the raw material solution and a 3.894mol/kg ammonia aqueous solution were simultaneously added into theaqueous solution, and thereafter, stirring was performed for 60 minuteswhile keeping the liquid temperature at 40° C., whereby a colloidalsolution was obtained (procedure 1, first neutralization step). Here,the raw material aqueous solution was continuously added over 240minutes at an addition rate of 10.8 g/min, and the ammonia aqueoussolution was continuously added over 240 minutes at an addition rate of10.8 g/min. While adding the raw material aqueous solution and theammonia aqueous solution, the ratio of the cumulative addition amount ofthe alkali to the total cumulative addition amount of the acid groupcontained in the raw material solution is 0.93 equivalents. The pH ofthe obtained colloidal solution was 2.1.

Subsequently, to the colloidal solution obtained by the procedure 1, 866g of a citric acid solution with a citric acid concentration of 1.041mol/kg as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 582 g of a 12.465 mol/kg ammonia solution wasadded thereto all at once, and then, the resulting mixture was held for10 minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding582 g of the ammonia solution all at once was 8.6.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 150 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 30.2 nm.

Subsequently, 1299 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 1680 g. The temperature was adjusted to 40° C. while stirring, and2085 g of 2-propanol was added thereto, followed by stirring for 5minutes, and thereafter, 1029 g of a 21.24 mass % ammonia aqueoussolution was added thereto. Further, 3936 g of tetraethoxysilane (TEOS)as a silicon compound having a hydrolyzable group was added thereto over35 minutes. Thereafter, while keeping the liquid temperature at 40° C.,stirring was continued as such for 1 hour to carry out coating with ahydrolysate of the silicon compound. Thereafter, the obtained solutionwas washed and subjected to solid-liquid separation, and the product wascollected as a cake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1060° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 80° C. andstirred for 20 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 1 mS/m or less.

Subsequently, to the washed slurry, a 35 mass % TEAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 35.0% was obtained. Here, the additionamount of the TEAOH aqueous solution was set so that the TEAOHconcentration in the surface modifier-containing slurry became 0.099mol/kg. In this case, the pH of the slurry became 13.

460 g of the obtained surface modifier-containing slurry was subjectedto an ultrasonic dispersion treatment for 1 hour with an ultrasoniccleaner (Yamato 5510 Branson, manufactured by (Yamato Scientific) Co.,Ltd.), and then subjected to a centrifugation treatment at a rotationspeed of 8800 rpm for 130 minutes with an R10A3 rotor of a centrifuge(himac CR21GII, manufactured by Hitachi Koki Co., Ltd.), and 250 g of asupernatant slurry containing fine particles was removed and a slurry onthe precipitate side was obtained. The gravitational acceleration in thecentrifugation treatment was set to 15,000 G. For the obtained slurry onthe precipitate side, the operation of addition of 250 g of a 0.099mol/kg TEAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 8800 rpm and 15,000 G for 130 minutes, and removal of 250 gof a supernatant slurry was repeated further 15 times, whereby a slurryon the precipitate side was obtained.

After 250 g of a 0.099 mol/kg TEAOH aqueous solution was added to 210 gof the obtained slurry on the precipitate side, the resulting mixturewas subjected to an ultrasonic dispersion treatment for 1 hour with anultrasonic cleaner (Yamato 5510 Branson, manufactured by (YamatoScientific) Co., Ltd.), and then subjected to a centrifugation treatmentat a rotation speed of 8800 rpm for 60 minutes with an R10A3 rotor of acentrifuge (himac CR21GII, manufactured by Hitachi Koki Co., Ltd.), and250 g of a supernatant slurry was collected. The gravitationalacceleration in the centrifugation treatment was set to 15,000 G. For anobtained slurry on the precipitate side, the operation of addition of250 g of a 0.099 mol/kg TEAOH aqueous solution, ultrasonic dispersion, acentrifugation treatment at 8800 rpm and 15,000 G for 60 minutes, andcollection of 250 g of a supernatant slurry was repeated further 12times, whereby a total of 3250 g of the supernatant slurry wascollected. To the collected supernatant slurry, a 1 mass % sulfuric acidaqueous solution was added to adjust the pH to 6.5 and aggregatesubstitution-type ε-iron oxide particles, followed by filtration througha membrane filter, and the solid was collected and then dried, wherebyan iron-based oxide magnetic powder according to Example 19 wasobtained.

The obtained iron-based oxide magnetic powder had a coercive force of353 (kA/m), a saturation magnetization of 15.8 (Am²/kg), a square ratioof 0.620, an SFD of 0.84, and an I_(L)/I_(H) value of 0.33. Further, theobtained iron-based oxide magnetic powder was dispersed in pure waterusing an ultrasonic disperser to prepare a slurry, and the preparedslurry was dropped onto a collodion film on a grid and adhered thereto,and then, naturally dried. Thereafter, carbon vapor deposition wasperformed, and the resultant was subjected to TEM observation. As aresult of measuring 200 particles by TEM observation, the TEM averageparticle diameter was 16.7 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.0%, the number ratio ofparticles with a particle diameter of 20 nm or more was 5.9%, thecoefficient of variation (CV value) was 12%, and the average circularityof the particles was 0.970. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol.

Example 20

As a raw material solution, 2586 g of an aqueous solution with anFe(III) concentration of 1.099 mol/kg, a Ga(III) concentration of 0.184mol/kg, a Co(II) concentration of 0.061 mol/kg, a Ti(IV) concentrationof 0.044 mol/kg, a chloride ion concentration of 3.408 mol/kg, and anitrate ion concentration of 0.673 mol/kg was prepared. Subsequently, ina 10 L reaction vessel, 4757 g of an aqueous solution with an Fe(III)concentration of 0.198 mol/kg and a chloride ion concentration of 0.594mol/kg was placed, and the liquid temperature was adjusted to 40° C.while mechanically stirring with a stirring blade. Subsequently, whilecontinuing stirring and keeping the liquid temperature at 40° C., theraw material solution and a 3.885 mol/kg ammonia aqueous solution weresimultaneously added into the aqueous solution, and thereafter, stirringwas performed for 60 minutes while keeping the liquid temperature at 40°C., whereby a colloidal solution was obtained (procedure 1, firstneutralization step). Here, the raw material aqueous solution wascontinuously added over 240 minutes at an addition rate of 10.7 g/min,and the ammonia aqueous solution was continuously added over 240 minutesat an addition rate of 10.7 g/min. While adding the raw material aqueoussolution and the ammonia aqueous solution, the ratio of the cumulativeaddition amount of the alkali to the total cumulative addition amount ofthe acid group contained in the raw material solution is 0.95equivalents. The pH of the obtained colloidal solution was 1.9.

Subsequently, to the colloidal solution obtained by the procedure 1, 866g of a citric acid solution with a citric acid concentration of 1.041mol/kg as a hydroxycarboxylic acid was continuously added over 60minutes under the condition of 40° C. (hydroxycarboxylic acid additionstep), and thereafter, 573 g of a 12.553 mol/kg ammonia solution wasadded thereto all at once, and then, the resulting mixture was held for10 minutes while stirring under the condition of temperature of 40° C.,whereby a slurry of a precursor, which is an intermediate product, wasobtained (procedure 2, second neutralization step). The pH after adding573 g of the ammonia solution all at once was 8.6.

The precursor slurry obtained by the procedure 2 was collected andwashed with an ultrafiltration membrane and a membrane having a UFmolecular weight cut-off of 50,000 until the electrical conductivity ofthe filtrate became 150 mS/m or less (procedure 3).

When the DLS average diameter of the precursor particles contained inthe washed slurry obtained by the procedure 3 was measured, the DLSaverage diameter thereof was 30.8 nm.

Subsequently, 1220 g of the washed slurry obtained by the procedure 3was taken out, and pure water was added thereto to give a total amountof 1680 g. The temperature was adjusted to 40° C. while stirring, and2085 g of 2-propanol was added thereto, followed by stirring for 5minutes, and thereafter, 1022 g of a 21.39 mass % ammonia aqueoussolution was added thereto. Further, 3936 g of tetraethoxysilane (TEAS)as a silicon compound having a hydrolyzable group was added thereto over35 minutes. Thereafter, while keeping the liquid temperature at 40° C.,stirring was continued as such for 1 hour to carry out coating with ahydrolysate of the silicon compound. Thereafter, the obtained solutionwas washed and subjected to solid-liquid separation, and the product wascollected as a cake (procedure 4).

After the cake (the precursor coated with the hydrolysate in a gel form)obtained by the procedure 4 was dried, the resulting dry powder wassubjected to a heat treatment at 1062° C. for 4 hours in a furnace withan air atmosphere, whereby a powder of iron oxide containing substituentmetal elements and coated with silicon oxide was obtained. Note that thehydrolysate of the silicon compound turns into an oxide when it issubjected to a heat treatment in an air atmosphere (procedure 5).

The powder of iron oxide containing substituent metal elements andcoated with silicon oxide obtained by the procedure 5 was placed in a 20mass % NaOH aqueous solution with a liquid temperature of 80° C. andstirred for 20 hours so as to carry out a treatment of removing thesilicon oxide on the surfaces of the particles, whereby a slurrycontaining particles of ε-iron oxide in which Fe sites were partiallysubstituted by other metal elements was obtained (procedure 6).

The slurry obtained by the procedure 6 was washed until the electricalconductivity became 1 mS/m or less.

Subsequently, to the washed slurry, a 35 mass % TEAOH aqueous solutionwas added as a surface modifier, whereby a surface modifier-containingslurry with a solid content of 40.0% was obtained. Here, the additionamount of the TEAOH aqueous solution was set so that the TEAOHconcentration in the surface modifier-containing slurry became 0.099mol/kg. In this case, the pH of the slurry became 13.

460 g of the obtained surface modifier-containing slurry was subjectedto an ultrasonic dispersion treatment for 1 hour with an ultrasoniccleaner (Yamato 5510 Branson, manufactured by (Yamato Scientific) Co.,Ltd.), and then subjected to a centrifugation treatment at a rotationspeed of 8800 rpm for 130 minutes with an R10A3 rotor of a centrifuge(himac CR21GII, manufactured by Hitachi Koki Co., Ltd.), and 250 g of asupernatant slurry containing fine particles was removed and a slurry onthe precipitate side was obtained. The gravitational acceleration in thecentrifugation treatment was set to 15,000 G. For the obtained slurry onthe precipitate side, the operation of addition of 250 g of a 0.099mol/kg TEAOH aqueous solution, ultrasonic dispersion, a centrifugationtreatment at 8800 rpm and 15,000 G for 130 minutes, and removal of 250 gof a supernatant slurry was repeated further 9 times, whereby a slurryon the precipitate side was obtained.

After 250 g of a 0.099 mol/kg TEAOH aqueous solution was added to 210 gof the obtained slurry on the precipitate side, the resulting mixturewas subjected to an ultrasonic dispersion treatment for 1 hour with anultrasonic cleaner (Yamato 5510 Branson, manufactured by (YamatoScientific) Co., Ltd.), and then subjected to a centrifugation treatmentat a rotation speed of 8800 rpm for 60 minutes with an R10A3 rotor of acentrifuge (himac CR21GII, manufactured by Hitachi Koki Co., Ltd.), and250 g of a supernatant slurry was collected. The gravitationalacceleration in the centrifugation treatment was set to 15,000 G. For anobtained slurry on the precipitate side, the operation of addition of250 g of a 0.099 mol/kg TEAOH aqueous solution, ultrasonic dispersion, acentrifugation treatment at 8800 rpm and 15,000 G for 60 minutes, andcollection of 250 g of a supernatant slurry was repeated further 12times, whereby a total of 3250 g of the supernatant slurry wascollected. To the collected supernatant slurry, a 1 mass % sulfuric acidaqueous solution was added to adjust the pH to 6.5 and aggregatesubstitution-type ε-iron oxide particles, followed by filtration througha membrane filter, and the solid was collected and then dried, wherebyan iron-based oxide magnetic powder according to Example 20 wasobtained.

The obtained iron-based oxide magnetic powder had a coercive force of327 (kA/m), a saturation magnetization of 15.4 (Am²/kg), a square ratioof 0.628, an SFD of 0.97, and an I_(L)/I_(H) value of 0.25. Further, theobtained iron-based oxide magnetic powder was dispersed in pure waterusing an ultrasonic disperser to prepare a slurry, and the preparedslurry was dropped onto a collodion film on a grid and adhered thereto,and then, naturally dried. Thereafter, carbon vapor deposition wasperformed, and the resultant was subjected to TEM observation. As aresult of measuring 200 particles by TEM observation, the TEM averageparticle diameter was 17.3 nm, the number ratio of particles with aparticle diameter of 8 nm or less was 0.0%, the number ratio ofparticles with a particle diameter of 20 nm or more was 12.9%, thecoefficient of variation (CV value) was 14%, and the average circularityof the particles was 0.969. In addition, evaluation of compositionalanalysis was performed, and the molar compositional ratio of each metalelement was calculated when the total amount of iron and the substituentmetal elements was set to 2 mol. The above measurement results are shownin Tables 3 and 4.

TABLE 1 Raw materials Preparation composition (molar ratio Fe + Ga +Co + Ti + Ni + Or = 2.0) Iron raw material Fe Ga Co Ti Ni Cr Example 1Iron(III) chloride 1.705 0.175 0.070 0.050 0 0 Example 2 Iron(III)chloride 1.710 0.200 0.045 0.045 0 0 Example 3 Iron(III) chloride 1.8350.075 0.045 0.045 0 0 Example 4 Iron(III) chloride 1.910 0.000 0.0450.045 0 0 Example 5 Iron(III) chloride 1.730 0.150 0.070 0.050 0 0Example 6 Iron(III) chloride 1.605 0.275 0.070 0.050 0 0 Example 7Iron(III) chloride 1.630 0.250 0.070 0.050 0 0 Example 8 Iron(III)chloride 1.530 0.350 0.070 0.050 0 0 Example 9 Iron(III) chloride 1.6700.240 0.045 0.045 0 0 Comparative Iron(III) chloride 1.530 0.350 0.0700.050 0 0 example 1 Comparative Iron(III) nitrate 1.635 0.265 0.0500.050 0 0 example 2 Comparative Iron(III) nitrate 1.530 0.350 0.0700.050 0 0 example 3 Example 10 Iron(III) chloride 2.000 0.000 0.0000.000 0 0 Example 11 Iron(III) chloride 1.910 0.000 0.090 0.000 0 0Example 12 Iron(III) chloride 1.910 0.000 0.000 0.090 0 0 Example 13Iron(III) chloride 1.888 0.000 0.068 0.045 0 0 Example 14 Iron(III)chloride + 1.530 0.400 0.070 0.000 0 0 iron(III) nitrate Example 15Iron(III) chloride 1.575 0.280 0.045 0.000 0.100 0 Example 16 Iron(III)chloride 1.575 0.280 0.045 0.000 0 0.100 Example 17 Iron(III) chloride1.530 0.350 0.070 0.050 0 0 Example 18 Iron(III) chloride 1.705 0.1750.070 0.050 0 0 Example 19 Iron(III) chloride 1.530 0.400 0.070 0.000 00 Example 20 Iron(III) chloride 1.670 0.210 0.070 0.050 0 0

TABLE 2 Production method Reaction First stage of neutralizationHydroxycarboxylic acid temper- Cumulative Neutrali- pH at end Metalmolar ature alkali addition zation of alkali ratio to addition (° C.)equivalent time (min) addition type amount Example 1 40 0.75 240 1.7citric acid 0.20 Example 2 40 0.80 240 1.9 citric acid 0.20 Example 3 400.75 240 2.1 citric acid 0.20 Example 4 40 0.75 240 2.0 citric acid 0.20Example 5 40 0.75 240 1.7 citric acid 0.20 Example 6 40 0.75 240 1.7citric acid 0.20 Example 7 40 0.75 240 1.6 citric acid 0.20 Example 8 400.80 240 1.9 citric acid 0.20 Example 9 40 0.79 240 1.8 citric acid 0.20Comparative 40 0.80 240 1.8 citric acid 0.20 example 1 Comparative 400.80 1 2.0 citric acid 0.15 example 2 Comparative 40 0.80 1 1.8 citricacid 0.20 example 3 Example 10 30 1.00 240 2.3 citric acid 0.20 Example11 40 0.99 240 2.1 citric acid 0.20 Example 12 40 0.99 240 1.8 citricacid 0.20 Example 13 40 0.99 240 1.6 citric acid 0.20 Example 14 40 0.93240 2.0 citric acid 0.20 Example 15 40 0.94 240 2.1 citric acid 0.19Example 16 40 0.94 240 2.0 citric acid 0.20 Example 17 40 0.80 240 1.8citric acid 0.20 Example 18 40 0.75 240 1.7 citric acid 0.20 Example 1940 0.93 240 2.1 citric acid 0.20 Example 20 40 0.95 240 1.9 citric acid0.20 Production method pH arrived at DLS average Firing Concentration pHof surface second stage diameter of temper- Type of of surface modifier-of precursor ature surface modifier containing neutralization (nm) (°C.) modifier (mol/kg) slurry Example 1 8.5 36 1130 TMAOH 0.065 13Example 2 8.6 48 1050 TMAOH 0.065 13 Example 3 8.9 36 1110 TMAOH 0.06513 Example 4 8.9 37 1110 TMAOH 0.065 13 Example 5 8.4 37 1130 TMAOH0.065 13 Example 6 8.5 33 1130 TMAOH 0.065 13 Example 7 8.4 32 1130TMAOH 0.065 13 Example 8 8.6 51 1116 TMAOH 0.065 13 Example 9 8.7 581050 TMAOH 0.065 13 Comparative 8.6 39 1130 — — — example 1 Comparative8.5 — 1070 — — — example 2 Comparative 8.6 31 971 TMAOH 0.065 13 example3 Example 10 9.2 128 1130 TEAOH 0.099 13 Example 11 9.2 147 1130 TEAOH0.099 13 Example 12 9.1 36 1130 TEAOH 0.099 13 Example 13 8.8 39 1130TEAOH 0.099 13 Example 14 8.6 196 1060 TEAOH 0.099 13 Example 15 8.8 401130 TEAOH 0.099 13 Example 16 8.5 354 1130 TEAOH 0.099 13 Example 178.8 46 1116 TMAOH 0.065 13 Example 18 8.5 38 1115 TMAOH 0.065 13 Example19 8.6 30 1060 TEAOH 0.099 13 Example 20 8.6 31 1062 TEAOH 0.099 13

TABLE 3 Iron-based oxide magnetic powder TEM observation Averageparticle Ratio of particles with Ratio of particles with Coefficient ofComposition (molar ratio: diameter size of 8 nm or less size of 20 nm ormore variation Fe + Ga + Co + Ti + Ni + Cr = 2.0) (nm) (% by number) (%by number) (%) Circularity Fe Ga Co Ti others Example 1 16.0 0.0 1.3 120.974 1.76 0.15 0.05 0.05 — Example 2 14.2 0.5 7.2 19 0.960 1.75 0.180.03 0.04 — Example 3 14.1 0.0 0.0 14 0.972 1.86 0.06 0.04 0.04 —Example 4 14.2 0.0 0.0 12 0.972 1.93 — 0.03 0.04 — Example 5 15.3 0.10.3 13 0.973 1.76 0.14 0.05 0.05 — Example 6 15.7 0.0 1.0 15 0.970 1.660.26 0.04 0.05 — Example 7 15.5 0.0 0.5 14 0.972 1.67 0.24 0.05 0.05 —Example 8 15.0 0.3 1.0 14 0.974 1.57 0.34 0.04 0.05 — Example 9 14.0 0.30.6 14 0.966 1.70 0.22 0.04 0.04 — Comparative 16.6 0.0 14.9 20 0.9721.59 0.32 0.04 0.05 — example 1 Comparative 16.6 5.8 26.0 40 0.919 1.670.24 0.04 0.05 — example 2 Comparative 18.8 3.7 38.9 29 0.949 1.56 0.350.04 0.05 — example 3 Example 10 10.3 1.2 0.0 9 0.970 2.00 — — — —Example 11 16.5 0.5 3.0 12 0.966 1.91 — 0.70 — — Example 12 15.9 0.0 1.612 0.974 1.92 — — 0.08 — Example 13 13.3 1.4 0.5 17 0.970 1.93 — 0.030.04 — Example 14 16.4 0.0 2.4 12 0.975 1.55 0.40 0.05 — — Example 1515.8 0.6 1.8 12 0.973 1.68 0.27 0.03 — Ni: 0.02 Example 16 15.8 0.0 2.013 0.975 1.63 0.26 0.03 — Cr: 0.08 Example 17 15.3 0.3 2.1 14 0.973 1.590.32 0.04 0.05 — Example 18 15.8 0.0 1.4 12 0.975 1.76 0.15 0.05 0.05 —Example 19 16.7 0.0 5.9 12 0.970 1.51 0.45 0.04 0.00 — Example 20 17.30.0 12.9 14 0.969 1.67 0.24 0.05 0.04 —

TABLE 4 Iron-based oxide magnetic powder Magnetic properties Hmax (kA/m)Hc (kA/m) σs (Am²/kg) SQ SFD (main peak) I_(L)/I_(H) Example 1 1035 31416.6 0.637 0.71 0.14 Example 2 1035 317 14.7 0.590 1.43 0.56 Example 31035 441 12.8 0.645 1.08 0.42 Example 4 1035 558 11.4 0.673 0.89 0.29Example 5 1035 341 16.2 0.641 0.69 0.14 Example 6 1035 200 17.4 0.5771.02 0.27 Example 7 1035 223 17.4 0.595 0.91 0.21 Example 8 1035 18317.7 0.558 1.19 0.32 Example 9 1035 272 15.0 0.606 1.16 0.34 Comparativeexample 1 1035 177 17.9 0.522 1.00 0.41 Comparative example 2 1035 31315.8 0.535 0.89 0.50 Comparative example 3 1035 237 15.8 0.560 0.83 0.25Example 10 2387 560 12.8 0.544 1.99 0.47 Example 11 2387 875 20.6 0.5021.66 0.50 Example 12 2387 1267 16.0 0.546 0.50 0.12 Example 13 2387 49914.8 0.529 1.78 0.46 Example 14 1035 324 16.9 0.626 0.77 0.20 Example 152387 505 22.4 0.494 0.72 0.59 Example 16 2387 471 16.1 0.542 0.78 0.18Example 17 1035 171 16.6 0.556 1.21 0.33 Example 18 1035 349 15.3 0.6430.71 0.15 Example 19 1035 353 15.8 0.620 0.84 0.33 Example 20 1035 32715.4 0.628 0.97 0.25

1. An iron-based oxide magnetic powder, comprising particles of ε-ironoxide or ε-iron oxide in which Fe sites are partially substituted byother metal elements, wherein an average particle diameter measured witha transmission electron microscope is 10 nm or more and 20 nm or less,the number ratio of particles with a particle diameter of 8 nm or lessis 3% or less, the number ratio of particles with a particle diameter of20 nm or more is 25% or less, the average circularity of particlesmeasured with a transmission electron microscope is 0.955 or more, andthe coefficient of variation of the particle diameter measured with atransmission electron microscope is 19% or less.
 2. The iron-based oxidemagnetic powder according to claim 1, wherein the metal element thatpartially substitutes the Fe sites is one type or two or more types ofGa, Co, and Ti.
 3. The iron-based oxide magnetic powder according toclaim 1, wherein the metal element that partially substitutes the Fesites is one type or two or more types of Ga, Co, Ti, Ni, Mn, Cr, Nd,Dy, and Gd.
 4. A method for producing an iron-based oxide magneticpowder composed of particles of ε-iron oxide or ε-iron oxide in which Fesites are partially substituted by other metal elements, comprising: araw material solution preparation step of preparing a raw materialsolution containing trivalent iron ions, or trivalent iron ions and ionsof a metal element that partially substitutes the Fe sites, and analkaline aqueous solution for neutralizing the raw material solution; afirst neutralization step of continuously or intermittently adding eachof the raw material solution and the alkaline aqueous solution to areaction system and mixing so as to adjust the pH of the reaction systemto 1.0 or higher and 3.0 or lower; a step of adding a hydroxycarboxylicacid to the aqueous solution after the first neutralization step; asecond neutralization step of neutralizing the pH to 7.0 or higher and10.0 or lower by adding an alkali to the aqueous solution to which thehydroxycarboxylic acid is added, thereby obtaining a slurry containing aprecipitate of an iron oxyhydroxide or a substituent metalelement-containing iron oxyhydroxide; a step of adding a siliconcompound having a hydrolyzable group to the slurry containing the ironoxyhydroxide or the substituent metal element-containing ironoxyhydroxide, thereby coating the iron oxyhydroxide or the substituentmetal element-containing iron oxyhydroxide with a hydrolysate of thesilicon compound; a step of heating the substituent metalelement-containing iron oxyhydroxide coated with the hydrolysate of thesilicon compound, thereby forming ε-iron oxide or ε-iron oxide, in whichFe sites are partially substituted by other metal elements, coated withsilicon oxide; a step of removing the silicon oxide on the ε-iron oxideor the ε-iron oxide, in which Fe sites are partially substituted byother metal elements, coated with the silicon oxide, thereby obtaining aslurry containing the ε-iron oxide or the ε-iron oxide, in which Fesites are partially substituted by other metal elements; a step ofadding a quaternary ammonium salt at a concentration of 0.009 mol/kg ormore and 1.0 mol/kg or less as a surface modifier to the slurrycontaining the ε-iron oxide or the ε-iron oxide, in which Fe sites arepartially substituted by other metal elements, and also adjusting the pHto 11.0 or higher and 14.0 or lower; a step of subjecting the surfacemodifier-containing slurry to a dispersion treatment, thereby obtaininga dispersion slurry of particles of the ε-iron oxide or the ε-ironoxide, in which Fe sites are partially substituted by other metalelements; and a step of classifying the dispersion slurry of theparticles of the ε-iron oxide or the ε-iron oxide, in which Fe sites arepartially substituted by other metal elements.
 5. The method forproducing an iron-based oxide magnetic powder according to claim 4,wherein the first neutralization step is a step of continuously orintermittently adding each of the raw material solution and the alkalineaqueous solution containing the alkali in an amount of 0.4 equivalentsor more and 0.9 equivalents or less with respect to the total amount ofan acid group contained in the raw material solution to the reactionsystem which does not contain trivalent iron ions or ions of a metalelement that partially substitutes the Fe sites and mixing so as toadjust the pH of the reaction system to 1.0 or higher and 3.0 or lower.6. The method for producing an iron-based oxide magnetic powderaccording to claim 4, wherein the first neutralization step is a step inwhich when each of the raw material solution and the alkaline aqueoussolution is continuously or intermittently added to the reaction systemwhich previously contains trivalent iron ions, or trivalent iron ionsand ions of a metal element that partially substitutes the Fe sites, theamount of the trivalent iron ions and the ions of the metal element thatpartially substitutes the Fe sites previously contained in the reactionsystem is set to 50 mol % or less of the sum of the amount of thetrivalent iron ions and the ions of the metal element that partiallysubstitutes the Fe sites and the amount of the trivalent iron ions andthe ions of the metal element that partially substitutes the Fe sitescontained in the raw material solution added to the reaction system, andeach of the raw material solution and the alkaline aqueous solutioncontaining the alkali in an amount of 0.4 equivalents or more and 1.8equivalents or less with respect to the total amount of an acid groupcontained in the raw material solution is continuously or intermittentlyadded to the reaction system and mixed so as to adjust the pH of thereaction system to 1.0 or higher and 3.0 or lower.
 7. The method forproducing an iron-based oxide magnetic powder according to claim 5,wherein in the first neutralization step, the addition rate of each ofthe raw material solution and the alkaline aqueous solution is adjustedso as to maintain the cumulative addition amount of the alkali withrespect to the total cumulative addition amount of the acid groupcontained in the raw material solution within a range of 0.4 equivalentsor more and 0.9 equivalents or less through the step.
 8. The method forproducing an iron-based oxide magnetic powder according to claim 4,wherein in the first neutralization step, the raw material solution andthe alkaline aqueous solution are added over 10 minutes or more.
 9. Themethod for producing an iron-based oxide magnetic powder according toclaim 4, wherein the average diameter measured with a dynamic lightscattering particle size distribution analyzer of the slurry containingthe iron oxyhydroxide or the substituent metal element-containing ironoxyhydroxide obtained in the second neutralization step is 300 nm orless.
 10. The method for producing an iron-based oxide magnetic powderaccording to claim 4, wherein iron(III) chloride is used as a supplysource of the trivalent iron ions contained in the raw materialsolution.
 11. The method for producing an iron-based oxide magneticpowder according to claim 5, wherein the cumulative addition amount ofthe alkali with respect to the total cumulative addition amount of theacid group contained in the raw material solution is set to 0.6equivalents or more and 0.9 equivalents or less through the firstneutralization step.
 12. The method for producing an iron-based oxidemagnetic powder according to claim 6, wherein the cumulative additionamount of the alkali with respect to the total cumulative additionamount of the acid group contained in the raw material solution is setto 0.4 equivalents or more and 1.8 equivalents or less through the firstneutralization step.